diff --git a/package.json b/package.json
index e62332ff1989..6f082e6cd173 100644
--- a/package.json
+++ b/package.json
@@ -26,8 +26,7 @@
     "codecov": "^1.0.1",
     "derequire": "^2.0.2",
     "eslint": "^3.9.0",
-    "eslint-config-babel": "^2.0.1",
-    "eslint-plugin-babel": "^3.3.0",
+    "eslint-config-babel": "^5.0.0",
     "eslint-plugin-flowtype": "^2.20.0",
     "flow-bin": "^0.34.0",
     "gulp": "^3.9.0",
diff --git a/packages/babel-cli/src/_babel-node.js b/packages/babel-cli/src/_babel-node.js
index 3c7c40ed4d80..40471639bd9d 100644
--- a/packages/babel-cli/src/_babel-node.js
+++ b/packages/babel-cli/src/_babel-node.js
@@ -11,7 +11,7 @@ import _ from "lodash";
 import "babel-polyfill";
 import register from "babel-register";
 
-let program = new commander.Command("babel-node");
+const program = new commander.Command("babel-node");
 
 program.option("-e, --eval [script]", "Evaluate script");
 program.option("-p, --print [code]", "Evaluate script and print result");
@@ -21,7 +21,7 @@ program.option("-x, --extensions [extensions]", "List of extensions to hook into
 program.option("-w, --plugins [string]", "", util.list);
 program.option("-b, --presets [string]", "", util.list);
 
-let pkg = require("../package.json");
+const pkg = require("../package.json");
 program.version(pkg.version);
 program.usage("[options] [ -e script | script.js ] [arguments]");
 program.parse(process.argv);
@@ -38,7 +38,7 @@ register({
 
 //
 
-let replPlugin = ({ types: t }) => ({
+const replPlugin = ({ types: t }) => ({
   visitor: {
     ModuleDeclaration(path) {
       throw path.buildCodeFrameError("Modules aren't supported in the REPL");
@@ -62,7 +62,7 @@ let replPlugin = ({ types: t }) => ({
 
 //
 
-let _eval = function (code, filename) {
+const _eval = function (code, filename) {
   code = code.trim();
   if (!code) return undefined;
 
@@ -84,7 +84,7 @@ if (program.eval || program.print) {
   global.__filename = "[eval]";
   global.__dirname = process.cwd();
 
-  let module = new Module(global.__filename);
+  const module = new Module(global.__filename);
   module.filename = global.__filename;
   module.paths    = Module._nodeModulePaths(global.__dirname);
 
@@ -92,9 +92,9 @@ if (program.eval || program.print) {
   global.module  = module;
   global.require = module.require.bind(module);
 
-  let result = _eval(code, global.__filename);
+  const result = _eval(code, global.__filename);
   if (program.print) {
-    let output = _.isString(result) ? result : inspect(result);
+    const output = _.isString(result) ? result : inspect(result);
     process.stdout.write(output + "\n");
   }
 } else {
@@ -111,7 +111,7 @@ if (program.eval || program.print) {
       }
 
       if (arg[0] === "-") {
-        let parsedArg = program[arg.slice(2)];
+        const parsedArg = program[arg.slice(2)];
         if (parsedArg && parsedArg !== true) {
           ignoreNext = true;
         }
@@ -123,7 +123,7 @@ if (program.eval || program.print) {
     args = args.slice(i);
 
     // make the filename absolute
-    let filename = args[0];
+    const filename = args[0];
     if (!pathIsAbsolute(filename)) args[0] = path.join(process.cwd(), filename);
 
     // add back on node and concat the sliced args
diff --git a/packages/babel-cli/src/babel-node.js b/packages/babel-cli/src/babel-node.js
index 0830a2313d43..e3d0c2e90196 100755
--- a/packages/babel-cli/src/babel-node.js
+++ b/packages/babel-cli/src/babel-node.js
@@ -5,8 +5,8 @@
  * when found, before invoking the "real" _babel-node(1) executable.
  */
 
-let getV8Flags = require("v8flags");
-let path = require("path");
+const getV8Flags = require("v8flags");
+const path = require("path");
 
 let args = [path.join(__dirname, "_babel-node")];
 
@@ -14,7 +14,7 @@ let babelArgs = process.argv.slice(2);
 let userArgs;
 
 // separate node arguments from script arguments
-let argSeparator = babelArgs.indexOf("--");
+const argSeparator = babelArgs.indexOf("--");
 if (argSeparator > -1) {
   userArgs  = babelArgs.slice(argSeparator); // including the  --
   babelArgs = babelArgs.slice(0, argSeparator);
@@ -75,13 +75,13 @@ getV8Flags(function (err, v8Flags) {
   }
 
   try {
-    let kexec = require("kexec");
+    const kexec = require("kexec");
     kexec(process.argv[0], args);
   } catch (err) {
     if (err.code !== "MODULE_NOT_FOUND") throw err;
 
-    let child_process = require("child_process");
-    let proc = child_process.spawn(process.argv[0], args, { stdio: "inherit" });
+    const child_process = require("child_process");
+    const proc = child_process.spawn(process.argv[0], args, { stdio: "inherit" });
     proc.on("exit", function (code, signal) {
       process.on("exit", function () {
         if (signal) {
diff --git a/packages/babel-cli/src/babel/dir.js b/packages/babel-cli/src/babel/dir.js
index 78be975c8885..f097874a2a0e 100644
--- a/packages/babel-cli/src/babel/dir.js
+++ b/packages/babel-cli/src/babel/dir.js
@@ -1,18 +1,18 @@
-let outputFileSync = require("output-file-sync");
-let slash          = require("slash");
-let path           = require("path");
-let util           = require("./util");
-let fs             = require("fs");
-let _              = require("lodash");
+const outputFileSync = require("output-file-sync");
+const slash          = require("slash");
+const path           = require("path");
+const util           = require("./util");
+const fs             = require("fs");
+const _              = require("lodash");
 
 module.exports = function (commander, filenames) {
   function write(src, relative) {
     // remove extension and then append back on .js
     relative = relative.replace(/\.(\w*?)$/, "") + ".js";
 
-    let dest = path.join(commander.outDir, relative);
+    const dest = path.join(commander.outDir, relative);
 
-    let data = util.compile(src, {
+    const data = util.compile(src, {
       sourceFileName: slash(path.relative(dest + "/..", src)),
       sourceMapTarget: path.basename(relative)
     });
@@ -20,7 +20,7 @@ module.exports = function (commander, filenames) {
 
     // we've requested explicit sourcemaps to be written to disk
     if (data.map && commander.sourceMaps && commander.sourceMaps !== "inline") {
-      let mapLoc = dest + ".map";
+      const mapLoc = dest + ".map";
       data.code = util.addSourceMappingUrl(data.code, mapLoc);
       outputFileSync(mapLoc, JSON.stringify(data.map));
     }
@@ -37,7 +37,7 @@ module.exports = function (commander, filenames) {
     if (util.canCompile(filename, commander.extensions)) {
       write(src, filename);
     } else if (commander.copyFiles) {
-      let dest = path.join(commander.outDir, filename);
+      const dest = path.join(commander.outDir, filename);
       outputFileSync(dest, fs.readFileSync(src));
       util.chmod(src, dest);
     }
@@ -46,13 +46,13 @@ module.exports = function (commander, filenames) {
   function handle(filename) {
     if (!fs.existsSync(filename)) return;
 
-    let stat = fs.statSync(filename);
+    const stat = fs.statSync(filename);
 
     if (stat.isDirectory(filename)) {
-      let dirname = filename;
+      const dirname = filename;
 
       _.each(util.readdir(dirname), function (filename) {
-        let src = path.join(dirname, filename);
+        const src = path.join(dirname, filename);
         handleFile(src, filename);
       });
     } else {
@@ -65,17 +65,17 @@ module.exports = function (commander, filenames) {
   }
 
   if (commander.watch) {
-    let chokidar = util.requireChokidar();
+    const chokidar = util.requireChokidar();
 
     _.each(filenames, function (dirname) {
-      let watcher = chokidar.watch(dirname, {
+      const watcher = chokidar.watch(dirname, {
         persistent: true,
         ignoreInitial: true
       });
 
       _.each(["add", "change"], function (type) {
         watcher.on(type, function (filename) {
-          let relative = path.relative(dirname, filename) || filename;
+          const relative = path.relative(dirname, filename) || filename;
           try {
             handleFile(filename, relative);
           } catch (err) {
diff --git a/packages/babel-cli/src/babel/file.js b/packages/babel-cli/src/babel/file.js
index 1feb8d2aadc3..bb1f6e949486 100644
--- a/packages/babel-cli/src/babel/file.js
+++ b/packages/babel-cli/src/babel/file.js
@@ -1,10 +1,10 @@
-let convertSourceMap = require("convert-source-map");
-let sourceMap        = require("source-map");
-let slash            = require("slash");
-let path             = require("path");
-let util             = require("./util");
-let fs               = require("fs");
-let _                = require("lodash");
+const convertSourceMap = require("convert-source-map");
+const sourceMap        = require("source-map");
+const slash            = require("slash");
+const path             = require("path");
+const util             = require("./util");
+const fs               = require("fs");
+const _                = require("lodash");
 
 module.exports = function (commander, filenames, opts) {
   if (commander.sourceMaps === "inline") {
@@ -13,8 +13,8 @@ module.exports = function (commander, filenames, opts) {
 
   let results = [];
 
-  let buildResult = function () {
-    let map = new sourceMap.SourceMapGenerator({
+  const buildResult = function () {
+    const map = new sourceMap.SourceMapGenerator({
       file: path.basename(commander.outFile || "") || "stdout",
       sourceRoot: opts.sourceRoot
     });
@@ -26,8 +26,8 @@ module.exports = function (commander, filenames, opts) {
       code += result.code + "\n";
 
       if (result.map) {
-        let consumer = new sourceMap.SourceMapConsumer(result.map);
-        let sources = new Set();
+        const consumer = new sourceMap.SourceMapConsumer(result.map);
+        const sources = new Set();
 
         consumer.eachMapping(function (mapping) {
           if (mapping.source != null) sources.add(mapping.source);
@@ -46,7 +46,7 @@ module.exports = function (commander, filenames, opts) {
         });
 
         sources.forEach((source) => {
-          let content = consumer.sourceContentFor(source, true);
+          const content = consumer.sourceContentFor(source, true);
           if (content !== null) {
             map.setSourceContent(source, content);
           }
@@ -68,13 +68,13 @@ module.exports = function (commander, filenames, opts) {
     };
   };
 
-  let output = function () {
-    let result = buildResult();
+  const output = function () {
+    const result = buildResult();
 
     if (commander.outFile) {
       // we've requested for a sourcemap to be written to disk
       if (commander.sourceMaps && commander.sourceMaps !== "inline") {
-        let mapLoc = commander.outFile + ".map";
+        const mapLoc = commander.outFile + ".map";
         result.code = util.addSourceMappingUrl(result.code, mapLoc);
         fs.writeFileSync(mapLoc, JSON.stringify(result.map));
       }
@@ -85,13 +85,13 @@ module.exports = function (commander, filenames, opts) {
     }
   };
 
-  let stdin = function () {
+  const stdin = function () {
     let code = "";
 
     process.stdin.setEncoding("utf8");
 
     process.stdin.on("readable", function () {
-      let chunk = process.stdin.read();
+      const chunk = process.stdin.read();
       if (chunk !== null) code += chunk;
     });
 
@@ -103,16 +103,16 @@ module.exports = function (commander, filenames, opts) {
     });
   };
 
-  let walk = function () {
-    let _filenames = [];
+  const walk = function () {
+    const _filenames = [];
     results = [];
 
     _.each(filenames, function (filename) {
       if (!fs.existsSync(filename)) return;
 
-      let stat = fs.statSync(filename);
+      const stat = fs.statSync(filename);
       if (stat.isDirectory()) {
-        let dirname = filename;
+        const dirname = filename;
 
         _.each(util.readdirFilter(filename), function (filename) {
           _filenames.push(path.join(dirname, filename));
@@ -131,7 +131,7 @@ module.exports = function (commander, filenames, opts) {
       }
       sourceFilename = slash(sourceFilename);
 
-      let data = util.compile(filename, {
+      const data = util.compile(filename, {
         sourceFileName: sourceFilename,
       });
 
@@ -142,14 +142,14 @@ module.exports = function (commander, filenames, opts) {
     output();
   };
 
-  let files = function () {
+  const files = function () {
 
     if (!commander.skipInitialBuild) {
       walk();
     }
 
     if (commander.watch) {
-      let chokidar = util.requireChokidar();
+      const chokidar = util.requireChokidar();
       chokidar.watch(filenames, {
         persistent: true,
         ignoreInitial: true
diff --git a/packages/babel-cli/src/babel/index.js b/packages/babel-cli/src/babel/index.js
index 236c2df72aae..0e1472d953a3 100755
--- a/packages/babel-cli/src/babel/index.js
+++ b/packages/babel-cli/src/babel/index.js
@@ -3,14 +3,14 @@
 
 require("babel-core");
 
-let fs         = require("fs");
-let commander  = require("commander");
-let kebabCase  = require("lodash/kebabCase");
-let options    = require("babel-core").options;
-let util       = require("babel-core").util;
-let uniq       = require("lodash/uniq");
-let each       = require("lodash/each");
-let glob       = require("glob");
+const fs         = require("fs");
+const commander  = require("commander");
+const kebabCase  = require("lodash/kebabCase");
+const options    = require("babel-core").options;
+const util       = require("babel-core").util;
+const uniq       = require("lodash/uniq");
+const each       = require("lodash/each");
+const glob       = require("glob");
 
 each(options, function (option, key) {
   if (option.hidden) return;
@@ -31,7 +31,7 @@ each(options, function (option, key) {
     arg = "-" + option.shorthand + ", " + arg;
   }
 
-  let desc = [];
+  const desc = [];
   if (option.deprecated) desc.push("[DEPRECATED] " + option.deprecated);
   if (option.description) desc.push(option.description);
 
@@ -46,7 +46,7 @@ commander.option("-d, --out-dir [out]", "Compile an input directory of modules i
 commander.option("-D, --copy-files", "When compiling a directory copy over non-compilable files");
 commander.option("-q, --quiet", "Don't log anything");
 
-let pkg = require("../../package.json");
+const pkg = require("../../package.json");
 commander.version(pkg.version + " (babel-core " + require("babel-core").version + ")");
 commander.usage("[options] <files ...>");
 commander.parse(process.argv);
@@ -59,7 +59,7 @@ if (commander.extensions) {
 
 //
 
-let errors = [];
+const errors = [];
 
 let filenames = commander.args.reduce(function (globbed, input) {
   let files = glob.sync(input);
@@ -104,7 +104,7 @@ if (errors.length) {
 
 //
 
-let opts = exports.opts = {};
+const opts = exports.opts = {};
 
 each(options, function (opt, key) {
   if (commander[key] !== undefined && commander[key] !== opt.default) {
diff --git a/packages/babel-cli/src/babel/util.js b/packages/babel-cli/src/babel/util.js
index fd06aa6ce78c..df8c72b3aa9e 100644
--- a/packages/babel-cli/src/babel/util.js
+++ b/packages/babel-cli/src/babel/util.js
@@ -1,11 +1,11 @@
-let commander = require("commander");
-let readdir   = require("fs-readdir-recursive");
-let index     = require("./index");
-let babel     = require("babel-core");
-let util      = require("babel-core").util;
-let path      = require("path");
-let fs        = require("fs");
-let _         = require("lodash");
+const commander = require("commander");
+const readdir   = require("fs-readdir-recursive");
+const index     = require("./index");
+const babel     = require("babel-core");
+const util      = require("babel-core").util;
+const path      = require("path");
+const fs        = require("fs");
+const _         = require("lodash");
 
 export function chmod(src, dest) {
   fs.chmodSync(dest, fs.statSync(src).mode);
@@ -19,7 +19,7 @@ export function readdirFilter(filename) {
 
 export { readdir };
 
-export let canCompile = util.canCompile;
+export const canCompile = util.canCompile;
 
 export function shouldIgnore(loc) {
   return util.shouldIgnore(loc, index.opts.ignore, index.opts.only);
@@ -37,7 +37,7 @@ export function transform(filename, code, opts) {
   opts = _.defaults(opts || {}, index.opts);
   opts.filename = filename;
 
-  let result = babel.transform(code, opts);
+  const result = babel.transform(code, opts);
   result.filename = filename;
   result.actual = code;
   return result;
@@ -45,7 +45,7 @@ export function transform(filename, code, opts) {
 
 export function compile(filename, opts) {
   try {
-    let code = fs.readFileSync(filename, "utf8");
+    const code = fs.readFileSync(filename, "utf8");
     return transform(filename, code, opts);
   } catch (err) {
     if (commander.watch) {
diff --git a/packages/babel-cli/test/index.js b/packages/babel-cli/test/index.js
index 61cb19aed949..9a5d9f0594a6 100644
--- a/packages/babel-cli/test/index.js
+++ b/packages/babel-cli/test/index.js
@@ -1,29 +1,29 @@
-let readdir        = require("fs-readdir-recursive");
-let helper         = require("babel-helper-fixtures");
-let assert         = require("assert");
-let rimraf         = require("rimraf");
-let outputFileSync = require("output-file-sync");
-let child          = require("child_process");
-let path           = require("path");
-let chai           = require("chai");
-let fs             = require("fs");
-let _              = require("lodash");
-
-let fixtureLoc = path.join(__dirname, "fixtures");
-let tmpLoc = path.join(__dirname, "tmp");
-
-let presetLocs = [
+const readdir        = require("fs-readdir-recursive");
+const helper         = require("babel-helper-fixtures");
+const assert         = require("assert");
+const rimraf         = require("rimraf");
+const outputFileSync = require("output-file-sync");
+const child          = require("child_process");
+const path           = require("path");
+const chai           = require("chai");
+const fs             = require("fs");
+const _              = require("lodash");
+
+const fixtureLoc = path.join(__dirname, "fixtures");
+const tmpLoc = path.join(__dirname, "tmp");
+
+const presetLocs = [
   path.join(__dirname, "../../babel-preset-es2015"),
   path.join(__dirname, "../../babel-preset-react")
 ].join(",");
 
-let pluginLocs = [
+const pluginLocs = [
   path.join(__dirname, "/../../babel-plugin-transform-strict-mode"),
   path.join(__dirname, "/../../babel-plugin-transform-es2015-modules-commonjs"),
 ].join(",");
 
-let readDir = function (loc) {
-  let files = {};
+const readDir = function (loc) {
+  const files = {};
   if (fs.existsSync(loc)) {
     _.each(readdir(loc), function (filename) {
       files[filename] = helper.readFile(path.join(loc, filename));
@@ -32,14 +32,14 @@ let readDir = function (loc) {
   return files;
 };
 
-let saveInFiles = function (files) {
+const saveInFiles = function (files) {
   _.each(files, function (content, filename) {
     outputFileSync(filename, content);
   });
 };
 
-let assertTest = function (stdout, stderr, opts) {
-  let expectStderr = opts.stderr.trim();
+const assertTest = function (stdout, stderr, opts) {
+  const expectStderr = opts.stderr.trim();
   stderr = stderr.trim();
 
   if (opts.stderr) {
@@ -52,7 +52,7 @@ let assertTest = function (stdout, stderr, opts) {
     throw new Error("stderr:\n" + stderr);
   }
 
-  let expectStdout = opts.stdout.trim();
+  const expectStdout = opts.stdout.trim();
   stdout = stdout.trim();
   stdout = stdout.replace(/\\/g, "/");
 
@@ -67,13 +67,13 @@ let assertTest = function (stdout, stderr, opts) {
   }
 
   _.each(opts.outFiles, function (expect, filename) {
-    let actual = helper.readFile(filename);
+    const actual = helper.readFile(filename);
     chai.expect(actual).to.equal(expect, "out-file " + filename);
   });
 };
 
-let buildTest = function (binName, testName, opts) {
-  let binLoc = path.join(__dirname, "../lib", binName);
+const buildTest = function (binName, testName, opts) {
+  const binLoc = path.join(__dirname, "../lib", binName);
 
   return function (callback) {
     clear();
@@ -91,7 +91,7 @@ let buildTest = function (binName, testName, opts) {
 
     args = args.concat(opts.args);
 
-    let spawn = child.spawn(process.execPath, args);
+    const spawn = child.spawn(process.execPath, args);
 
     let stderr = "";
     let stdout = "";
@@ -127,7 +127,7 @@ let buildTest = function (binName, testName, opts) {
   };
 };
 
-let clear = function () {
+const clear = function () {
   process.chdir(__dirname);
   if (fs.existsSync(tmpLoc)) rimraf.sync(tmpLoc);
   fs.mkdirSync(tmpLoc);
@@ -137,22 +137,22 @@ let clear = function () {
 _.each(fs.readdirSync(fixtureLoc), function (binName) {
   if (binName[0] === ".") return;
 
-  let suiteLoc = path.join(fixtureLoc, binName);
+  const suiteLoc = path.join(fixtureLoc, binName);
   describe("bin/" + binName, function () {
     _.each(fs.readdirSync(suiteLoc), function (testName) {
       if (testName[0] === ".") return;
 
-      let testLoc = path.join(suiteLoc, testName);
+      const testLoc = path.join(suiteLoc, testName);
 
-      let opts = {
+      const opts = {
         args: []
       };
 
-      let optionsLoc = path.join(testLoc, "options.json");
+      const optionsLoc = path.join(testLoc, "options.json");
       if (fs.existsSync(optionsLoc)) _.merge(opts, require(optionsLoc));
 
       _.each(["stdout", "stdin", "stderr"], function (key) {
-        let loc = path.join(testLoc, key + ".txt");
+        const loc = path.join(testLoc, key + ".txt");
         if (fs.existsSync(loc)) {
           opts[key] = helper.readFile(loc);
         } else {
@@ -163,7 +163,7 @@ _.each(fs.readdirSync(fixtureLoc), function (binName) {
       opts.outFiles = readDir(path.join(testLoc, "out-files"));
       opts.inFiles  = readDir(path.join(testLoc, "in-files"));
 
-      let babelrcLoc = path.join(testLoc, ".babelrc");
+      const babelrcLoc = path.join(testLoc, ".babelrc");
       if (fs.existsSync(babelrcLoc)) {
         // copy .babelrc file to tmp directory
         opts.inFiles[".babelrc"] = helper.readFile(babelrcLoc);
diff --git a/packages/babel-code-frame/src/index.js b/packages/babel-code-frame/src/index.js
index 1e570ab69259..9476ef617d22 100644
--- a/packages/babel-code-frame/src/index.js
+++ b/packages/babel-code-frame/src/index.js
@@ -46,8 +46,8 @@ const BRACKET = /^[()\[\]{}]$/;
  */
 
 function getTokenType(match) {
-  let [offset, text] = match.slice(-2);
-  let token = matchToToken(match);
+  const [offset, text] = match.slice(-2);
+  const token = matchToToken(match);
 
   if (token.type === "name") {
     if (esutils.keyword.isReservedWordES6(token.value)) {
@@ -79,8 +79,8 @@ function getTokenType(match) {
 
 function highlight(defs: Object, text: string) {
   return text.replace(jsTokens, function (...args) {
-    let type = getTokenType(args);
-    let colorize = defs[type];
+    const type = getTokenType(args);
+    const colorize = defs[type];
     if (colorize) {
       return args[0].split(NEWLINE).map((str) => colorize(str)).join("\n");
     } else {
@@ -101,21 +101,21 @@ export default function (
 ): string {
   colNumber = Math.max(colNumber, 0);
 
-  let highlighted = (opts.highlightCode && Chalk.supportsColor) || opts.forceColor;
+  const highlighted = (opts.highlightCode && Chalk.supportsColor) || opts.forceColor;
   let chalk = Chalk;
   if (opts.forceColor) {
     chalk = new Chalk.constructor({ enabled: true });
   }
-  let maybeHighlight = (chalkFn, string) => {
+  const maybeHighlight = (chalkFn, string) => {
     return highlighted ? chalkFn(string) : string;
   };
-  let defs = getDefs(chalk);
+  const defs = getDefs(chalk);
   if (highlighted) rawLines = highlight(defs, rawLines);
 
-  let linesAbove = opts.linesAbove || 2;
-  let linesBelow = opts.linesBelow || 3;
+  const linesAbove = opts.linesAbove || 2;
+  const linesBelow = opts.linesBelow || 3;
 
-  let lines = rawLines.split(NEWLINE);
+  const lines = rawLines.split(NEWLINE);
   let start = Math.max(lineNumber - (linesAbove + 1), 0);
   let end   = Math.min(lines.length, lineNumber + linesBelow);
 
@@ -124,16 +124,16 @@ export default function (
     end = lines.length;
   }
 
-  let numberMaxWidth = String(end).length;
+  const numberMaxWidth = String(end).length;
 
-  let frame = lines.slice(start, end).map((line, index) => {
-    let number = start + 1 + index;
-    let paddedNumber = ` ${number}`.slice(-numberMaxWidth);
-    let gutter = ` ${paddedNumber} | `;
+  const frame = lines.slice(start, end).map((line, index) => {
+    const number = start + 1 + index;
+    const paddedNumber = ` ${number}`.slice(-numberMaxWidth);
+    const gutter = ` ${paddedNumber} | `;
     if (number === lineNumber) {
       let markerLine = "";
       if (colNumber) {
-        let markerSpacing = line.slice(0, colNumber - 1).replace(/[^\t]/g, " ");
+        const markerSpacing = line.slice(0, colNumber - 1).replace(/[^\t]/g, " ");
         markerLine = [
           "\n ",
           maybeHighlight(defs.gutter, gutter.replace(/\d/g, " ")),
diff --git a/packages/babel-code-frame/test/index.js b/packages/babel-code-frame/test/index.js
index c85d59ddec60..b00b88386db4 100644
--- a/packages/babel-code-frame/test/index.js
+++ b/packages/babel-code-frame/test/index.js
@@ -1,6 +1,6 @@
-let assert = require("assert");
-let chalk = require("chalk");
-let codeFrame = require("..");
+const assert = require("assert");
+const chalk = require("chalk");
+const codeFrame = require("..");
 
 describe("babel-code-frame", function () {
   it("basic usage", function () {
@@ -119,7 +119,7 @@ describe("babel-code-frame", function () {
   });
 
   it("opts.linesAbove", function () {
-    let rawLines = [
+    const rawLines = [
       "/**",
       " * Sums two numbers.",
       " *",
@@ -143,7 +143,7 @@ describe("babel-code-frame", function () {
   });
 
   it("opts.linesBelow", function () {
-    let rawLines = [
+    const rawLines = [
       "/**",
       " * Sums two numbers.",
       " *",
@@ -166,7 +166,7 @@ describe("babel-code-frame", function () {
   });
 
   it("opts.linesAbove and opts.linesBelow", function () {
-    let rawLines = [
+    const rawLines = [
       "/**",
       " * Sums two numbers.",
       " *",
@@ -188,10 +188,10 @@ describe("babel-code-frame", function () {
   });
 
   it("opts.forceColor", function() {
-    let marker = chalk.red.bold;
-    let gutter = chalk.grey;
+    const marker = chalk.red.bold;
+    const gutter = chalk.grey;
 
-    let rawLines = [
+    const rawLines = [
       "",
       "",
       "",
diff --git a/packages/babel-core/src/api/browser.js b/packages/babel-core/src/api/browser.js
index 1c53951acf38..d5d89f6b71d9 100644
--- a/packages/babel-core/src/api/browser.js
+++ b/packages/babel-core/src/api/browser.js
@@ -29,16 +29,16 @@ export function run(code: string, opts: Object = {}): any {
 export function load(url: string, callback: Function, opts: Object = {}, hold?: boolean) {
   opts.filename = opts.filename || url;
 
-  let xhr = global.ActiveXObject ? new global.ActiveXObject("Microsoft.XMLHTTP") : new global.XMLHttpRequest();
+  const xhr = global.ActiveXObject ? new global.ActiveXObject("Microsoft.XMLHTTP") : new global.XMLHttpRequest();
   xhr.open("GET", url, true);
   if ("overrideMimeType" in xhr) xhr.overrideMimeType("text/plain");
 
   xhr.onreadystatechange = function () {
     if (xhr.readyState !== 4) return;
 
-    let status = xhr.status;
+    const status = xhr.status;
     if (status === 0 || status === 200) {
-      let param = [xhr.responseText, opts];
+      const param = [xhr.responseText, opts];
       if (!hold) run(param);
       if (callback) callback(param);
     } else {
@@ -50,8 +50,8 @@ export function load(url: string, callback: Function, opts: Object = {}, hold?:
 }
 
 function runScripts() {
-  let scripts: Array<Array<any> | Object> = [];
-  let types   = ["text/ecmascript-6", "text/6to5", "text/babel", "module"];
+  const scripts: Array<Array<any> | Object> = [];
+  const types   = ["text/ecmascript-6", "text/6to5", "text/babel", "module"];
   let index   = 0;
 
   /**
@@ -59,7 +59,7 @@ function runScripts() {
    */
 
   function exec() {
-    let param = scripts[index];
+    const param = scripts[index];
     if (param instanceof Array) {
       run(param, index);
       index++;
@@ -72,7 +72,7 @@ function runScripts() {
    */
 
   function run(script: Object, i: number) {
-    let opts = {};
+    const opts = {};
 
     if (script.src) {
       load(script.src, function (param) {
@@ -87,10 +87,10 @@ function runScripts() {
 
   // Collect scripts with Babel `types`.
 
-  let _scripts = global.document.getElementsByTagName("script");
+  const _scripts = global.document.getElementsByTagName("script");
 
   for (let i = 0; i < _scripts.length; ++i) {
-    let _script = _scripts[i];
+    const _script = _scripts[i];
     if (types.indexOf(_script.type) >= 0) scripts.push(_script);
   }
 
diff --git a/packages/babel-core/src/api/node.js b/packages/babel-core/src/api/node.js
index a9e4792465d3..de1cfea682cc 100644
--- a/packages/babel-core/src/api/node.js
+++ b/packages/babel-core/src/api/node.js
@@ -31,10 +31,10 @@ export function Plugin(alias) {
 import Pipeline from "../transformation/pipeline";
 export { Pipeline };
 
-let pipeline = new Pipeline;
-export let analyse = pipeline.analyse.bind(pipeline);
-export let transform = pipeline.transform.bind(pipeline);
-export let transformFromAst = pipeline.transformFromAst.bind(pipeline);
+const pipeline = new Pipeline;
+export const analyse = pipeline.analyse.bind(pipeline);
+export const transform = pipeline.transform.bind(pipeline);
+export const transformFromAst = pipeline.transformFromAst.bind(pipeline);
 
 export function transformFile(filename: string, opts?: Object, callback: Function) {
   if (isFunction(opts)) {
diff --git a/packages/babel-core/src/helpers/get-possible-preset-names.js b/packages/babel-core/src/helpers/get-possible-preset-names.js
index b9fb1726bd06..a2140b9887fa 100644
--- a/packages/babel-core/src/helpers/get-possible-preset-names.js
+++ b/packages/babel-core/src/helpers/get-possible-preset-names.js
@@ -1,11 +1,11 @@
 export default function getPossiblePresetNames(presetName: string): Array<string> {
-  let possibleNames = [`babel-preset-${presetName}`, presetName];
+  const possibleNames = [`babel-preset-${presetName}`, presetName];
 
   // trying to resolve @organization shortcat
   // @foo/es2015 -> @foo/babel-preset-es2015
-  let matches = presetName.match(/^(@[^/]+)\/(.+)$/);
+  const matches = presetName.match(/^(@[^/]+)\/(.+)$/);
   if (matches) {
-    let [, orgName, presetPath] = matches;
+    const [, orgName, presetPath] = matches;
     possibleNames.push(`${orgName}/babel-preset-${presetPath}`);
   }
 
diff --git a/packages/babel-core/src/helpers/merge.js b/packages/babel-core/src/helpers/merge.js
index 5a695f6afa1d..1a01e22889c7 100644
--- a/packages/babel-core/src/helpers/merge.js
+++ b/packages/babel-core/src/helpers/merge.js
@@ -5,9 +5,9 @@ export default function (dest?: Object, src?: Object): ?Object {
 
   return mergeWith(dest, src, function (a, b) {
     if (b && Array.isArray(a)) {
-      let newArray = b.slice(0);
+      const newArray = b.slice(0);
 
-      for (let item of a) {
+      for (const item of a) {
         if (newArray.indexOf(item) < 0) {
           newArray.push(item);
         }
diff --git a/packages/babel-core/src/helpers/resolve.js b/packages/babel-core/src/helpers/resolve.js
index 623a910cbf38..e8493ac8f599 100644
--- a/packages/babel-core/src/helpers/resolve.js
+++ b/packages/babel-core/src/helpers/resolve.js
@@ -1,7 +1,7 @@
 import Module from "module";
 import path from "path";
 
-let relativeModules = {};
+const relativeModules = {};
 
 export default function (loc: string, relative: string = process.cwd()): ?string {
   // we're in the browser, probably
@@ -18,7 +18,7 @@ export default function (loc: string, relative: string = process.cwd()): ?string
     // Node presumes "." is process.cwd(), not our relative path.
     // Since this fake module is never "loaded", we don't have to worry about mutating
     // any global Node module cache state here.
-    let filename = path.join(relative, ".babelrc");
+    const filename = path.join(relative, ".babelrc");
     relativeMod.id = filename;
     relativeMod.filename = filename;
 
diff --git a/packages/babel-core/src/store.js b/packages/babel-core/src/store.js
index fab3d10f341d..1b172efbe045 100644
--- a/packages/babel-core/src/store.js
+++ b/packages/babel-core/src/store.js
@@ -15,7 +15,7 @@ export default class Store extends Map {
       return super.get(key);
     } else {
       if (Object.prototype.hasOwnProperty.call(this.dynamicData, key)) {
-        let val =  this.dynamicData[key]();
+        const val =  this.dynamicData[key]();
         this.set(key, val);
         return val;
       }
diff --git a/packages/babel-core/src/tools/build-external-helpers.js b/packages/babel-core/src/tools/build-external-helpers.js
index a26febd34bbe..237daf532c49 100644
--- a/packages/babel-core/src/tools/build-external-helpers.js
+++ b/packages/babel-core/src/tools/build-external-helpers.js
@@ -7,7 +7,7 @@ import template from "babel-template";
 import each from "lodash/each";
 import * as t from "babel-types";
 
-let buildUmdWrapper = template(`
+const buildUmdWrapper = template(`
   (function (root, factory) {
     if (typeof define === "function" && define.amd) {
       define(AMD_ARGUMENTS, factory);
@@ -22,9 +22,9 @@ let buildUmdWrapper = template(`
 `);
 
 function buildGlobal(namespace, builder) {
-  let body      = [];
-  let container = t.functionExpression(null, [t.identifier("global")], t.blockStatement(body));
-  let tree      = t.program([t.expressionStatement(t.callExpression(container, [helpers.get("selfGlobal")]))]);
+  const body      = [];
+  const container = t.functionExpression(null, [t.identifier("global")], t.blockStatement(body));
+  const tree      = t.program([t.expressionStatement(t.callExpression(container, [helpers.get("selfGlobal")]))]);
 
   body.push(t.variableDeclaration("var", [
     t.variableDeclarator(
@@ -39,7 +39,7 @@ function buildGlobal(namespace, builder) {
 }
 
 function buildUmd(namespace, builder) {
-  let body = [];
+  const body = [];
   body.push(t.variableDeclaration("var", [
     t.variableDeclarator(namespace, t.identifier("global"))
   ]));
@@ -63,7 +63,7 @@ function buildUmd(namespace, builder) {
 }
 
 function buildVar(namespace, builder) {
-  let body = [];
+  const body = [];
   body.push(t.variableDeclaration("var", [
     t.variableDeclarator(namespace, t.objectExpression([]))
   ]));
@@ -76,7 +76,7 @@ function buildHelpers(body, namespace, whitelist) {
   each(helpers.list, function (name) {
     if (whitelist && whitelist.indexOf(name) < 0) return;
 
-    let key = t.identifier(name);
+    const key = t.identifier(name);
     body.push(t.expressionStatement(
       t.assignmentExpression("=", t.memberExpression(namespace, key), helpers.get(name))
     ));
@@ -86,15 +86,15 @@ export default function (
   whitelist?: Array<string>,
   outputType: "global" | "umd" | "var" = "global",
 ) {
-  let namespace = t.identifier("babelHelpers");
+  const namespace = t.identifier("babelHelpers");
 
-  let builder = function (body) {
+  const builder = function (body) {
     return buildHelpers(body, namespace, whitelist);
   };
 
   let tree;
 
-  let build = {
+  const build = {
     global: buildGlobal,
     umd:    buildUmd,
     var:    buildVar,
diff --git a/packages/babel-core/src/transformation/file/index.js b/packages/babel-core/src/transformation/file/index.js
index 01d5b7efc9d3..a013eb5d6a43 100644
--- a/packages/babel-core/src/transformation/file/index.js
+++ b/packages/babel-core/src/transformation/file/index.js
@@ -32,9 +32,9 @@ const INTERNAL_PLUGINS = [
   [shadowFunctionsPlugin]
 ];
 
-let errorVisitor = {
+const errorVisitor = {
   enter(path, state) {
-    let loc = path.node.loc;
+    const loc = path.node.loc;
     if (loc) {
       state.loc = loc;
       path.stop();
@@ -69,7 +69,7 @@ export default class File extends Store {
       // All the "per preset" options are inherited from the main options.
       this.perPresetOpts = [];
       this.opts.presets.forEach((presetOpts) => {
-        let perPresetOpts = Object.assign(Object.create(this.opts), presetOpts);
+        const perPresetOpts = Object.assign(Object.create(this.opts), presetOpts);
         this.perPresetOpts.push(perPresetOpts);
         this.buildPluginsForOptions(perPresetOpts);
       });
@@ -125,7 +125,7 @@ export default class File extends Store {
 
   getMetadata() {
     let has = false;
-    for (let node of (this.ast.program.body: Array<Object>)) {
+    for (const node of (this.ast.program.body: Array<Object>)) {
       if (t.isModuleDeclaration(node)) {
         has = true;
         break;
@@ -165,7 +165,7 @@ export default class File extends Store {
       filenameRelative: opts.filename
     });
 
-    let basenameRelative = path.basename(opts.filenameRelative);
+    const basenameRelative = path.basename(opts.filenameRelative);
 
     defaults(opts, {
       sourceFileName:   basenameRelative,
@@ -180,13 +180,13 @@ export default class File extends Store {
       return;
     }
 
-    let plugins: Array<[PluginPass, Object]> = opts.plugins.concat(INTERNAL_PLUGINS);
-    let currentPluginVisitors = [];
-    let currentPluginPasses = [];
+    const plugins: Array<[PluginPass, Object]> = opts.plugins.concat(INTERNAL_PLUGINS);
+    const currentPluginVisitors = [];
+    const currentPluginPasses = [];
 
     // init plugins!
-    for (let ref of plugins) {
-      let [plugin, pluginOpts] = ref; // todo: fix - can't embed in loop head because of flow bug
+    for (const ref of plugins) {
+      const [plugin, pluginOpts] = ref; // todo: fix - can't embed in loop head because of flow bug
 
       currentPluginVisitors.push(plugin.visitor);
       currentPluginPasses.push(new PluginPass(this, plugin, pluginOpts));
@@ -201,7 +201,7 @@ export default class File extends Store {
   }
 
   getModuleName(): ?string {
-    let opts = this.opts;
+    const opts = this.opts;
     if (!opts.moduleIds) {
       return null;
     }
@@ -224,7 +224,7 @@ export default class File extends Store {
 
     if (opts.sourceRoot != null) {
       // remove sourceRoot from filename
-      let sourceRootRegEx = new RegExp("^" + opts.sourceRoot + "\/?");
+      const sourceRootRegEx = new RegExp("^" + opts.sourceRoot + "\/?");
       filenameRelative = filenameRelative.replace(sourceRootRegEx, "");
     }
 
@@ -245,20 +245,20 @@ export default class File extends Store {
   }
 
   resolveModuleSource(source: string): string {
-    let resolveModuleSource = this.opts.resolveModuleSource;
+    const resolveModuleSource = this.opts.resolveModuleSource;
     if (resolveModuleSource) source = resolveModuleSource(source, this.opts.filename);
     return source;
   }
 
   addImport(source: string, imported: string, name?: string = imported): Object {
-    let alias = `${source}:${imported}`;
+    const alias = `${source}:${imported}`;
     let id = this.dynamicImportIds[alias];
 
     if (!id) {
       source = this.resolveModuleSource(source);
       id = this.dynamicImportIds[alias] = this.scope.generateUidIdentifier(name);
 
-      let specifiers = [];
+      const specifiers = [];
 
       if (imported === "*") {
         specifiers.push(t.importNamespaceSpecifier(id));
@@ -268,7 +268,7 @@ export default class File extends Store {
         specifiers.push(t.importSpecifier(id, t.identifier(imported)));
       }
 
-      let declar = t.importDeclaration(specifiers, t.stringLiteral(source));
+      const declar = t.importDeclaration(specifiers, t.stringLiteral(source));
       declar._blockHoist = 3;
 
       this.path.unshiftContainer("body", declar);
@@ -278,7 +278,7 @@ export default class File extends Store {
   }
 
   addHelper(name: string): Object {
-    let declar = this.declarations[name];
+    const declar = this.declarations[name];
     if (declar) return declar;
 
     if (!this.usedHelpers[name]) {
@@ -286,17 +286,17 @@ export default class File extends Store {
       this.usedHelpers[name] = true;
     }
 
-    let generator = this.get("helperGenerator");
-    let runtime   = this.get("helpersNamespace");
+    const generator = this.get("helperGenerator");
+    const runtime   = this.get("helpersNamespace");
     if (generator) {
-      let res = generator(name);
+      const res = generator(name);
       if (res) return res;
     } else if (runtime) {
       return t.memberExpression(runtime, t.identifier(name));
     }
 
-    let ref = getHelper(name);
-    let uid = this.declarations[name] = this.scope.generateUidIdentifier(name);
+    const ref = getHelper(name);
+    const uid = this.declarations[name] = this.scope.generateUidIdentifier(name);
 
     if (t.isFunctionExpression(ref) && !ref.id) {
       ref.body._compact = true;
@@ -323,18 +323,18 @@ export default class File extends Store {
   ): Object {
     // Generate a unique name based on the string literals so we dedupe
     // identical strings used in the program.
-    let stringIds = raw.elements.map(function(string) {
+    const stringIds = raw.elements.map(function(string) {
       return string.value;
     });
-    let name = `${helperName}_${raw.elements.length}_${stringIds.join(",")}`;
+    const name = `${helperName}_${raw.elements.length}_${stringIds.join(",")}`;
 
-    let declar = this.declarations[name];
+    const declar = this.declarations[name];
     if (declar) return declar;
 
-    let uid = this.declarations[name] = this.scope.generateUidIdentifier("templateObject");
+    const uid = this.declarations[name] = this.scope.generateUidIdentifier("templateObject");
 
-    let helperId = this.addHelper(helperName);
-    let init = t.callExpression(helperId, [strings, raw]);
+    const helperId = this.addHelper(helperName);
+    const init = t.callExpression(helperId, [strings, raw]);
     init._compact = true;
     this.scope.push({
       id: uid,
@@ -345,9 +345,9 @@ export default class File extends Store {
   }
 
   buildCodeFrameError(node: Object, msg: string, Error: typeof Error = SyntaxError): Error {
-    let loc = node && (node.loc || node._loc);
+    const loc = node && (node.loc || node._loc);
 
-    let err = new Error(msg);
+    const err = new Error(msg);
 
     if (loc) {
       err.loc = loc.start;
@@ -367,13 +367,13 @@ export default class File extends Store {
   }
 
   mergeSourceMap(map: Object) {
-    let inputMap = this.opts.inputSourceMap;
+    const inputMap = this.opts.inputSourceMap;
 
     if (inputMap) {
-      let inputMapConsumer   = new sourceMap.SourceMapConsumer(inputMap);
-      let outputMapConsumer  = new sourceMap.SourceMapConsumer(map);
+      const inputMapConsumer   = new sourceMap.SourceMapConsumer(inputMap);
+      const outputMapConsumer  = new sourceMap.SourceMapConsumer(map);
 
-      let mergedGenerator = new sourceMap.SourceMapGenerator({
+      const mergedGenerator = new sourceMap.SourceMapGenerator({
         file: inputMapConsumer.file,
         sourceRoot: inputMapConsumer.sourceRoot
       });
@@ -402,7 +402,7 @@ export default class File extends Store {
         }
       });
 
-      let mergedMap = mergedGenerator.toJSON();
+      const mergedMap = mergedGenerator.toJSON();
       inputMap.mappings = mergedMap.mappings;
       return inputMap;
     } else {
@@ -419,8 +419,8 @@ export default class File extends Store {
 
       if (parserOpts.parser) {
         if (typeof parserOpts.parser === "string") {
-          let dirname = path.dirname(this.opts.filename) || process.cwd();
-          let parser = resolve(parserOpts.parser, dirname);
+          const dirname = path.dirname(this.opts.filename) || process.cwd();
+          const parser = resolve(parserOpts.parser, dirname);
           if (parser) {
             parseCode = require(parser).parse;
           } else {
@@ -439,7 +439,7 @@ export default class File extends Store {
     }
 
     this.log.debug("Parse start");
-    let ast = parseCode(code, parserOpts || this.parserOpts);
+    const ast = parseCode(code, parserOpts || this.parserOpts);
     this.log.debug("Parse stop");
     return ast;
   }
@@ -472,7 +472,7 @@ export default class File extends Store {
       this.log.debug("Start transform traverse");
 
       // merge all plugin visitors into a single visitor
-      let visitor = traverse.visitors.merge(this.pluginVisitors[i], pluginPasses, this.opts.wrapPluginVisitorMethod);
+      const visitor = traverse.visitors.merge(this.pluginVisitors[i], pluginPasses, this.opts.wrapPluginVisitorMethod);
       traverse(this.ast, visitor, this.scope);
 
       this.log.debug("End transform traverse");
@@ -500,7 +500,7 @@ export default class File extends Store {
 
       let message = err.message = `${this.opts.filename}: ${err.message}`;
 
-      let loc = err.loc;
+      const loc = err.loc;
       if (loc) {
         err.codeFrame = codeFrame(code, loc.line, loc.column + 1, this.opts);
         message += "\n" + err.codeFrame;
@@ -513,7 +513,7 @@ export default class File extends Store {
       }
 
       if (err.stack) {
-        let newStack = err.stack.replace(err.message, message);
+        const newStack = err.stack.replace(err.message, message);
         err.stack = newStack;
       }
 
@@ -529,28 +529,28 @@ export default class File extends Store {
 
   parseCode() {
     this.parseShebang();
-    let ast = this.parse(this.code);
+    const ast = this.parse(this.code);
     this.addAst(ast);
   }
 
   shouldIgnore() {
-    let opts = this.opts;
+    const opts = this.opts;
     return util.shouldIgnore(opts.filename, opts.ignore, opts.only);
   }
 
   call(key: "pre" | "post", pluginPasses: Array<PluginPass>) {
-    for (let pass of pluginPasses) {
-      let plugin = pass.plugin;
-      let fn = plugin[key];
+    for (const pass of pluginPasses) {
+      const plugin = pass.plugin;
+      const fn = plugin[key];
       if (fn) fn.call(pass, this);
     }
   }
 
   parseInputSourceMap(code: string): string {
-    let opts = this.opts;
+    const opts = this.opts;
 
     if (opts.inputSourceMap !== false) {
-      let inputMap = convertSourceMap.fromSource(code);
+      const inputMap = convertSourceMap.fromSource(code);
       if (inputMap) {
         opts.inputSourceMap = inputMap.toObject();
         code = convertSourceMap.removeComments(code);
@@ -561,7 +561,7 @@ export default class File extends Store {
   }
 
   parseShebang() {
-    let shebangMatch = shebangRegex.exec(this.code);
+    const shebangMatch = shebangRegex.exec(this.code);
     if (shebangMatch) {
       this.shebang = shebangMatch[0];
       this.code = this.code.replace(shebangRegex, "");
@@ -569,7 +569,7 @@ export default class File extends Store {
   }
 
   makeResult({ code, map, ast, ignored }: BabelFileResult): BabelFileResult {
-    let result = {
+    const result = {
       metadata: null,
       options:  this.opts,
       ignored:  !!ignored,
@@ -594,10 +594,10 @@ export default class File extends Store {
   }
 
   generate(): BabelFileResult {
-    let opts = this.opts;
-    let ast  = this.ast;
+    const opts = this.opts;
+    const ast  = this.ast;
 
-    let result: BabelFileResult = { ast };
+    const result: BabelFileResult = { ast };
     if (!opts.code) return this.makeResult(result);
 
     let gen = generate;
@@ -605,8 +605,8 @@ export default class File extends Store {
       gen = opts.generatorOpts.generator;
 
       if (typeof gen === "string") {
-        let dirname = path.dirname(this.opts.filename) || process.cwd();
-        let generator = resolve(gen, dirname);
+        const dirname = path.dirname(this.opts.filename) || process.cwd();
+        const generator = resolve(gen, dirname);
         if (generator) {
           gen = require(generator).print;
         } else {
@@ -617,7 +617,7 @@ export default class File extends Store {
 
     this.log.debug("Generation start");
 
-    let _result = gen(ast, opts.generatorOpts ? Object.assign(opts, opts.generatorOpts) : opts, this.code);
+    const _result = gen(ast, opts.generatorOpts ? Object.assign(opts, opts.generatorOpts) : opts, this.code);
     result.code = _result.code;
     result.map  = _result.map;
 
diff --git a/packages/babel-core/src/transformation/file/logger.js b/packages/babel-core/src/transformation/file/logger.js
index 3573c968e4f9..31bf92ac62ad 100644
--- a/packages/babel-core/src/transformation/file/logger.js
+++ b/packages/babel-core/src/transformation/file/logger.js
@@ -1,10 +1,10 @@
 import type File from "./index";
 import buildDebug from "debug/node";
 
-let verboseDebug = buildDebug("babel:verbose");
-let generalDebug = buildDebug("babel");
+const verboseDebug = buildDebug("babel:verbose");
+const generalDebug = buildDebug("babel");
 
-let seenDeprecatedMessages = [];
+const seenDeprecatedMessages = [];
 
 export default class Logger {
   constructor(file: File, filename: string) {
diff --git a/packages/babel-core/src/transformation/file/metadata.js b/packages/babel-core/src/transformation/file/metadata.js
index 0183006ffbfb..83b3580819d7 100644
--- a/packages/babel-core/src/transformation/file/metadata.js
+++ b/packages/babel-core/src/transformation/file/metadata.js
@@ -1,28 +1,28 @@
 import * as t from "babel-types";
 
-export let ModuleDeclaration = {
+export const ModuleDeclaration = {
   enter(path, file) {
-    let { node } = path;
+    const { node } = path;
     if (node.source) {
       node.source.value = file.resolveModuleSource(node.source.value);
     }
   }
 };
 
-export let ImportDeclaration = {
+export const ImportDeclaration = {
   exit(path, file) {
-    let { node } = path;
+    const { node } = path;
 
-    let specifiers = [];
-    let imported = [];
+    const specifiers = [];
+    const imported = [];
     file.metadata.modules.imports.push({
       source: node.source.value,
       imported,
       specifiers
     });
 
-    for (let specifier of (path.get("specifiers"): Array<Object>)) {
-      let local = specifier.node.local.name;
+    for (const specifier of (path.get("specifiers"): Array<Object>)) {
+      const local = specifier.node.local.name;
 
       if (specifier.isImportDefaultSpecifier()) {
         imported.push("default");
@@ -34,7 +34,7 @@ export let ImportDeclaration = {
       }
 
       if (specifier.isImportSpecifier()) {
-        let importedName = specifier.node.imported.name;
+        const importedName = specifier.node.imported.name;
         imported.push(importedName);
         specifiers.push({
           kind: "named",
@@ -55,18 +55,18 @@ export let ImportDeclaration = {
 };
 
 export function ExportDeclaration(path, file) {
-  let { node } = path;
+  const { node } = path;
 
-  let source = node.source ? node.source.value : null;
-  let exports = file.metadata.modules.exports;
+  const source = node.source ? node.source.value : null;
+  const exports = file.metadata.modules.exports;
 
   // export function foo() {}
   // export let foo = "bar";
-  let declar = path.get("declaration");
+  const declar = path.get("declaration");
   if (declar.isStatement()) {
-    let bindings = declar.getBindingIdentifiers();
+    const bindings = declar.getBindingIdentifiers();
 
-    for (let name in bindings) {
+    for (const name in bindings) {
       exports.exported.push(name);
       exports.specifiers.push({
         kind: "local",
@@ -77,8 +77,8 @@ export function ExportDeclaration(path, file) {
   }
 
   if (path.isExportNamedDeclaration() && node.specifiers) {
-    for (let specifier of (node.specifiers: Array<Object>)) {
-      let exported = specifier.exported.name;
+    for (const specifier of (node.specifiers: Array<Object>)) {
+      const exported = specifier.exported.name;
       exports.exported.push(exported);
 
       // export foo from "bar";
@@ -100,7 +100,7 @@ export function ExportDeclaration(path, file) {
         });
       }
 
-      let local = specifier.local;
+      const local = specifier.local;
       if (!local) continue;
 
       // export { foo } from "bar";
diff --git a/packages/babel-core/src/transformation/file/options/build-config-chain.js b/packages/babel-core/src/transformation/file/options/build-config-chain.js
index 02ce7376413a..f546f45b326a 100644
--- a/packages/babel-core/src/transformation/file/options/build-config-chain.js
+++ b/packages/babel-core/src/transformation/file/options/build-config-chain.js
@@ -6,15 +6,15 @@ import isAbsolute from "path-is-absolute";
 import path from "path";
 import fs from "fs";
 
-let existsCache = {};
-let jsonCache   = {};
+const existsCache = {};
+const jsonCache   = {};
 
 const BABELIGNORE_FILENAME = ".babelignore";
 const BABELRC_FILENAME     = ".babelrc";
 const PACKAGE_FILENAME     = "package.json";
 
 function exists(filename) {
-  let cached = existsCache[filename];
+  const cached = existsCache[filename];
   if (cached == null) {
     return existsCache[filename] = fs.existsSync(filename);
   } else {
@@ -23,8 +23,8 @@ function exists(filename) {
 }
 
 export default function buildConfigChain(opts: Object = {}, log?: Logger) {
-  let filename = opts.filename;
-  let builder = new ConfigChainBuilder(log);
+  const filename = opts.filename;
+  const builder = new ConfigChainBuilder(log);
 
   // resolve all .babelrc files
   if (opts.babelrc !== false) {
@@ -59,20 +59,20 @@ class ConfigChainBuilder {
 
     while (loc !== (loc = path.dirname(loc))) {
       if (!foundConfig) {
-        let configLoc = path.join(loc, BABELRC_FILENAME);
+        const configLoc = path.join(loc, BABELRC_FILENAME);
         if (exists(configLoc)) {
           this.addConfig(configLoc);
           foundConfig = true;
         }
 
-        let pkgLoc = path.join(loc, PACKAGE_FILENAME);
+        const pkgLoc = path.join(loc, PACKAGE_FILENAME);
         if (!foundConfig && exists(pkgLoc)) {
           foundConfig = this.addConfig(pkgLoc, "babel", JSON);
         }
       }
 
       if (!foundIgnore) {
-        let ignoreLoc = path.join(loc, BABELIGNORE_FILENAME);
+        const ignoreLoc = path.join(loc, BABELIGNORE_FILENAME);
         if (exists(ignoreLoc)) {
           this.addIgnoreConfig(ignoreLoc);
           foundIgnore = true;
@@ -84,7 +84,7 @@ class ConfigChainBuilder {
   }
 
   addIgnoreConfig(loc) {
-    let file  = fs.readFileSync(loc, "utf8");
+    const file  = fs.readFileSync(loc, "utf8");
     let lines = file.split("\n");
 
     lines = lines
@@ -107,7 +107,7 @@ class ConfigChainBuilder {
 
     this.resolvedConfigs.push(loc);
 
-    let content = fs.readFileSync(loc, "utf8");
+    const content = fs.readFileSync(loc, "utf8");
     let options;
 
     try {
@@ -144,7 +144,7 @@ class ConfigChainBuilder {
 
     // add extends clause
     if (options.extends) {
-      let extendsLoc = resolve(options.extends, dirname);
+      const extendsLoc = resolve(options.extends, dirname);
       if (extendsLoc) {
         this.addConfig(extendsLoc);
       } else {
@@ -162,7 +162,7 @@ class ConfigChainBuilder {
 
     // env
     let envOpts;
-    let envKey = process.env.BABEL_ENV || process.env.NODE_ENV || "development";
+    const envKey = process.env.BABEL_ENV || process.env.NODE_ENV || "development";
     if (options.env) {
       envOpts = options.env[envKey];
       delete options.env;
diff --git a/packages/babel-core/src/transformation/file/options/index.js b/packages/babel-core/src/transformation/file/options/index.js
index 69792cc65eac..c5f8a73c5175 100644
--- a/packages/babel-core/src/transformation/file/options/index.js
+++ b/packages/babel-core/src/transformation/file/options/index.js
@@ -4,7 +4,7 @@ import config from "./config";
 export { config };
 
 export function normaliseOptions(options: Object = {}): Object {
-  for (let key in options) {
+  for (const key in options) {
     let val = options[key];
     if (val == null) continue;
 
@@ -12,7 +12,7 @@ export function normaliseOptions(options: Object = {}): Object {
     if (opt && opt.alias) opt = config[opt.alias];
     if (!opt) continue;
 
-    let parser = parsers[opt.type];
+    const parser = parsers[opt.type];
     if (parser) val = parser(val);
 
     options[key] = val;
diff --git a/packages/babel-core/src/transformation/file/options/option-manager.js b/packages/babel-core/src/transformation/file/options/option-manager.js
index ed0eab6a4884..c4736355a6a0 100644
--- a/packages/babel-core/src/transformation/file/options/option-manager.js
+++ b/packages/babel-core/src/transformation/file/options/option-manager.js
@@ -53,7 +53,7 @@ export default class OptionManager {
   }>;
 
   static memoisePluginContainer(fn, loc, i, alias) {
-    for (let cache of (OptionManager.memoisedPlugins: Array<Object>)) {
+    for (const cache of (OptionManager.memoisedPlugins: Array<Object>)) {
       if (cache.container === fn) return cache.plugin;
     }
 
@@ -66,7 +66,7 @@ export default class OptionManager {
     }
 
     if (typeof obj === "object") {
-      let plugin = new Plugin(obj, alias);
+      const plugin = new Plugin(obj, alias);
       OptionManager.memoisedPlugins.push({
         container: fn,
         plugin: plugin
@@ -78,10 +78,10 @@ export default class OptionManager {
   }
 
   static createBareOptions() {
-    let opts = {};
+    const opts = {};
 
-    for (let key in config) {
-      let opt = config[key];
+    for (const key in config) {
+      const opt = config[key];
       opts[key] = clone(opt.default);
     }
 
@@ -120,11 +120,11 @@ export default class OptionManager {
         plugin = val;
       }
 
-      let alias = typeof plugin === "string" ? plugin : `${loc}$${i}`;
+      const alias = typeof plugin === "string" ? plugin : `${loc}$${i}`;
 
       // allow plugins to be specified as strings
       if (typeof plugin === "string") {
-        let pluginLoc = resolvePlugin(plugin, dirname);
+        const pluginLoc = resolvePlugin(plugin, dirname);
         if (pluginLoc) {
           plugin = require(pluginLoc);
         } else {
@@ -164,7 +164,7 @@ export default class OptionManager {
     }
 
     //
-    let opts = cloneDeepWith(rawOpts, (val) => {
+    const opts = cloneDeepWith(rawOpts, (val) => {
       if (val instanceof Plugin) {
         return val;
       }
@@ -174,16 +174,16 @@ export default class OptionManager {
     dirname = dirname || process.cwd();
     loc = loc || alias;
 
-    for (let key in opts) {
-      let option = config[key];
+    for (const key in opts) {
+      const option = config[key];
 
       // check for an unknown option
       if (!option && this.log) {
         if (removed[key]) {
           this.log.error(`Using removed Babel 5 option: ${alias}.${key} - ${removed[key].message}`, ReferenceError);
         } else {
-          let unknownOptErr = `Unknown option: ${alias}.${key}. Check out http://babeljs.io/docs/usage/options/ for more information about options.`;
-          let presetConfigErr = "A common cause of this error is the presence of a configuration options object without the corresponding preset name. Example:\n\nInvalid:\n  `{ presets: [{option: value}] }`\nValid:\n  `{ presets: [['presetName', {option: value}]] }`\n\nFor more detailed information on preset configuration, please see http://babeljs.io/docs/plugins/#pluginpresets-options.";
+          const unknownOptErr = `Unknown option: ${alias}.${key}. Check out http://babeljs.io/docs/usage/options/ for more information about options.`;
+          const presetConfigErr = "A common cause of this error is the presence of a configuration options object without the corresponding preset name. Example:\n\nInvalid:\n  `{ presets: [{option: value}] }`\nValid:\n  `{ presets: [['presetName', {option: value}]] }`\n\nFor more detailed information on preset configuration, please see http://babeljs.io/docs/plugins/#pluginpresets-options.";
 
           this.log.error(`${unknownOptErr}\n\n${presetConfigErr}`, ReferenceError);
         }
@@ -316,11 +316,11 @@ export default class OptionManager {
   }
 
   normaliseOptions() {
-    let opts = this.options;
+    const opts = this.options;
 
-    for (let key in config) {
-      let option = config[key];
-      let val    = opts[key];
+    for (const key in config) {
+      const option = config[key];
+      const val    = opts[key];
 
       // optional
       if (!val && option.optional) continue;
@@ -335,7 +335,7 @@ export default class OptionManager {
   }
 
   init(opts: Object = {}): Object {
-    for (let config of buildConfigChain(opts, this.log)) {
+    for (const config of buildConfigChain(opts, this.log)) {
       this.mergeOptions(config);
     }
 
diff --git a/packages/babel-core/src/transformation/file/options/parsers.js b/packages/babel-core/src/transformation/file/options/parsers.js
index fa8dbae260c9..2dcf5b7a71ac 100644
--- a/packages/babel-core/src/transformation/file/options/parsers.js
+++ b/packages/babel-core/src/transformation/file/options/parsers.js
@@ -1,7 +1,7 @@
 import slash from "slash";
 import * as util from "../../../util";
 
-export let filename = slash;
+export const filename = slash;
 
 export function boolean(val: any): boolean {
   return !!val;
diff --git a/packages/babel-core/src/transformation/internal-plugins/block-hoist.js b/packages/babel-core/src/transformation/internal-plugins/block-hoist.js
index de8bedf5bbfc..d546b9abd5bd 100644
--- a/packages/babel-core/src/transformation/internal-plugins/block-hoist.js
+++ b/packages/babel-core/src/transformation/internal-plugins/block-hoist.js
@@ -20,7 +20,7 @@ export default new Plugin({
       exit({ node }) {
         let hasChange = false;
         for (let i = 0; i < node.body.length; i++) {
-          let bodyNode = node.body[i];
+          const bodyNode = node.body[i];
           if (bodyNode && bodyNode._blockHoist != null) {
             hasChange = true;
             break;
diff --git a/packages/babel-core/src/transformation/internal-plugins/shadow-functions.js b/packages/babel-core/src/transformation/internal-plugins/shadow-functions.js
index 572b9bb37967..6b5995fc27d2 100644
--- a/packages/babel-core/src/transformation/internal-plugins/shadow-functions.js
+++ b/packages/babel-core/src/transformation/internal-plugins/shadow-functions.js
@@ -41,10 +41,10 @@ function shouldShadow(path, shadowPath) {
 
 function remap(path, key) {
   // ensure that we're shadowed
-  let shadowPath = path.inShadow(key);
+  const shadowPath = path.inShadow(key);
   if (!shouldShadow(path, shadowPath)) return;
 
-  let shadowFunction = path.node._shadowedFunctionLiteral;
+  const shadowFunction = path.node._shadowedFunctionLiteral;
 
   let currentFunction;
   let passedShadowFunction = false;
@@ -91,15 +91,15 @@ function remap(path, key) {
   // binding since arrow function syntax already does that.
   if (!passedShadowFunction) return;
 
-  let cached = fnPath.getData(key);
+  const cached = fnPath.getData(key);
   if (cached) return path.replaceWith(cached);
 
-  let id   = path.scope.generateUidIdentifier(key);
+  const id   = path.scope.generateUidIdentifier(key);
 
   fnPath.setData(key, id);
 
-  let classPath = fnPath.findParent((p) => p.isClass());
-  let hasSuperClass = !!(classPath && classPath.node && classPath.node.superClass);
+  const classPath = fnPath.findParent((p) => p.isClass());
+  const hasSuperClass = !!(classPath && classPath.node && classPath.node.superClass);
 
   if (key === "this" && fnPath.isMethod({kind: "constructor"}) && hasSuperClass) {
     fnPath.scope.push({ id });
diff --git a/packages/babel-core/src/transformation/pipeline.js b/packages/babel-core/src/transformation/pipeline.js
index 12b1edbba3e5..15b3f0cffddd 100644
--- a/packages/babel-core/src/transformation/pipeline.js
+++ b/packages/babel-core/src/transformation/pipeline.js
@@ -11,7 +11,7 @@ export default class Pipeline {
   }
 
   pretransform(code: string, opts?: Object): BabelFileResult {
-    let file = new File(opts, this);
+    const file = new File(opts, this);
     return file.wrap(code, function () {
       file.addCode(code);
       file.parseCode(code);
@@ -20,7 +20,7 @@ export default class Pipeline {
   }
 
   transform(code: string, opts?: Object): BabelFileResult {
-    let file = new File(opts, this);
+    const file = new File(opts, this);
     return file.wrap(code, function () {
       file.addCode(code);
       file.parseCode(code);
@@ -40,7 +40,7 @@ export default class Pipeline {
   transformFromAst(ast: Object, code: string, opts: Object): BabelFileResult {
     ast = normalizeAst(ast);
 
-    let file = new File(opts, this);
+    const file = new File(opts, this);
     return file.wrap(code, function () {
       file.addCode(code);
       file.addAst(ast);
diff --git a/packages/babel-core/src/transformation/plugin.js b/packages/babel-core/src/transformation/plugin.js
index afca4c882bf9..f3461ae9bf0d 100644
--- a/packages/babel-core/src/transformation/plugin.js
+++ b/packages/babel-core/src/transformation/plugin.js
@@ -31,7 +31,7 @@ export default class Plugin extends Store {
   visitor: Object;
 
   take(key) {
-    let val = this.raw[key];
+    const val = this.raw[key];
     delete this.raw[key];
     return val;
   }
@@ -40,13 +40,13 @@ export default class Plugin extends Store {
     if (!target[key]) return this[key];
     if (!this[key]) return target[key];
 
-    let fns: Array<?Function> = [target[key], this[key]];
+    const fns: Array<?Function> = [target[key], this[key]];
 
     return function (...args) {
       let val;
-      for (let fn of fns) {
+      for (const fn of fns) {
         if (fn) {
-          let ret = fn.apply(this, args);
+          const ret = fn.apply(this, args);
           if (ret != null) val = ret;
         }
       }
@@ -77,13 +77,13 @@ export default class Plugin extends Store {
 
     this.maybeInherit(loc);
 
-    for (let key in this.raw) {
+    for (const key in this.raw) {
       throw new Error(messages.get("pluginInvalidProperty", loc, i, key));
     }
   }
 
   normaliseVisitor(visitor: Object): Object {
-    for (let key of GLOBAL_VISITOR_PROPS) {
+    for (const key of GLOBAL_VISITOR_PROPS) {
       if (visitor[key]) {
         throw new Error("Plugins aren't allowed to specify catch-all enter/exit handlers. Please target individual nodes.");
       }
diff --git a/packages/babel-core/src/util.js b/packages/babel-core/src/util.js
index 4da7503e8cbf..97726e264016 100644
--- a/packages/babel-core/src/util.js
+++ b/packages/babel-core/src/util.js
@@ -15,8 +15,8 @@ export { inherits, inspect } from "util";
  */
 
 export function canCompile(filename: string, altExts?: Array<string>): boolean {
-  let exts = altExts || canCompile.EXTENSIONS;
-  let ext = path.extname(filename);
+  const exts = altExts || canCompile.EXTENSIONS;
+  const ext = path.extname(filename);
   return includes(exts, ext);
 }
 
@@ -63,7 +63,7 @@ export function regexify(val: any): RegExp {
     if (startsWith(val, "./") || startsWith(val, "*/")) val = val.slice(2);
     if (startsWith(val, "**/")) val = val.slice(3);
 
-    let regex = minimatch.makeRe(val, { nocase: true });
+    const regex = minimatch.makeRe(val, { nocase: true });
     return new RegExp(regex.source.slice(1, -1), "i");
   }
 
@@ -119,12 +119,12 @@ export function shouldIgnore(
   filename = filename.replace(/\\/g, "/");
 
   if (only) {
-    for (let pattern of only) {
+    for (const pattern of only) {
       if (_shouldIgnore(pattern, filename)) return false;
     }
     return true;
   } else if (ignore.length) {
-    for (let pattern of ignore) {
+    for (const pattern of ignore) {
       if (_shouldIgnore(pattern, filename)) return true;
     }
   }
diff --git a/packages/babel-core/test/api.js b/packages/babel-core/test/api.js
index 0b997fb4a025..70823258e5cf 100644
--- a/packages/babel-core/test/api.js
+++ b/packages/babel-core/test/api.js
@@ -1,9 +1,9 @@
-let babel                = require("../lib/api/node");
-let buildExternalHelpers = require("../lib/tools/build-external-helpers");
-let sourceMap            = require("source-map");
-let assert               = require("assert");
-let Plugin               = require("../lib/transformation/plugin");
-let generator            = require("babel-generator").default;
+const babel                = require("../lib/api/node");
+const buildExternalHelpers = require("../lib/tools/build-external-helpers");
+const sourceMap            = require("source-map");
+const assert               = require("assert");
+const Plugin               = require("../lib/transformation/plugin");
+const generator            = require("babel-generator").default;
 
 function assertIgnored(result) {
   assert.ok(result.ignored);
@@ -23,7 +23,7 @@ function transformAsync(code, opts) {
 }
 
 describe("parser and generator options", function() {
-  let recast = {
+  const recast = {
     parse: function(code, opts) {
       return opts.parser.parse(code);
     },
@@ -46,13 +46,13 @@ describe("parser and generator options", function() {
   }
 
   it("options", function() {
-    let string = "original;";
+    const string = "original;";
     assert.deepEqual(newTransform(string).ast, babel.transform(string).ast);
     assert.equal(newTransform(string).code, string);
   });
 
   it("experimental syntax", function() {
-    let experimental = "var a: number = 1;";
+    const experimental = "var a: number = 1;";
 
     assert.deepEqual(newTransform(experimental).ast, babel.transform(experimental, {
       parserOpts: {
@@ -82,7 +82,7 @@ describe("parser and generator options", function() {
   });
 
   it("other options", function() {
-    let experimental = "if (true) {\n  import a from 'a';\n}";
+    const experimental = "if (true) {\n  import a from 'a';\n}";
 
     assert.notEqual(newTransform(experimental).ast, babel.transform(experimental, {
       parserOpts: {
@@ -199,7 +199,7 @@ describe("api", function () {
               new Plugin({
                 visitor: {
                   Function: function(path) {
-                    let alias = path.scope.getProgramParent().path.get("body")[0].node;
+                    const alias = path.scope.getProgramParent().path.get("body")[0].node;
                     if (!babel.types.isTypeAlias(alias)) return;
 
                     // In case of `passPerPreset` being `false`, the
@@ -264,7 +264,7 @@ describe("api", function () {
   });
 
   it("source map merging", function () {
-    let result = babel.transform([
+    const result = babel.transform([
       "function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError(\"Cannot call a class as a function\"); } }",
       "",
       "let Foo = function Foo() {",
@@ -288,7 +288,7 @@ describe("api", function () {
       "};"
     ].join("\n"), result.code);
 
-    let consumer = new sourceMap.SourceMapConsumer(result.map);
+    const consumer = new sourceMap.SourceMapConsumer(result.map);
 
     assert.deepEqual(consumer.originalPositionFor({
       line: 7,
@@ -318,7 +318,7 @@ describe("api", function () {
       auxiliaryCommentBefore: "before",
       auxiliaryCommentAfter: "after",
       plugins: [function (babel) {
-        let t = babel.types;
+        const t = babel.types;
         return {
           visitor: {
             Program: function (path) {
@@ -539,8 +539,8 @@ describe("api", function () {
   });
 
   describe("env option", function () {
-    let oldBabelEnv = process.env.BABEL_ENV;
-    let oldNodeEnv = process.env.NODE_ENV;
+    const oldBabelEnv = process.env.BABEL_ENV;
+    const oldNodeEnv = process.env.NODE_ENV;
 
     setup(function () {
       // Tests need to run with the default and specific values for these. They
@@ -555,7 +555,7 @@ describe("api", function () {
     });
 
     it("default", function () {
-      let result = babel.transform("foo;", {
+      const result = babel.transform("foo;", {
         env: {
           development: { code: false }
         }
@@ -566,7 +566,7 @@ describe("api", function () {
 
     it("BABEL_ENV", function () {
       process.env.BABEL_ENV = "foo";
-      let result = babel.transform("foo;", {
+      const result = babel.transform("foo;", {
         env: {
           foo: { code: false }
         }
@@ -576,7 +576,7 @@ describe("api", function () {
 
     it("NODE_ENV", function () {
       process.env.NODE_ENV = "foo";
-      let result = babel.transform("foo;", {
+      const result = babel.transform("foo;", {
         env: {
           foo: { code: false }
         }
@@ -586,8 +586,8 @@ describe("api", function () {
   });
 
   it("resolveModuleSource option", function () {
-    let actual = "import foo from \"foo-import-default\";\nimport \"foo-import-bare\";\nexport { foo } from \"foo-export-named\";";
-    let expected = "import foo from \"resolved/foo-import-default\";\nimport \"resolved/foo-import-bare\";\nexport { foo } from \"resolved/foo-export-named\";";
+    const actual = "import foo from \"foo-import-default\";\nimport \"foo-import-bare\";\nexport { foo } from \"foo-export-named\";";
+    const expected = "import foo from \"resolved/foo-import-default\";\nimport \"resolved/foo-import-bare\";\nexport { foo } from \"resolved/foo-export-named\";";
 
     return transformAsync(actual, {
       resolveModuleSource: function (originalSource) {
@@ -600,25 +600,25 @@ describe("api", function () {
 
   describe("buildExternalHelpers", function () {
     it("all", function () {
-      let script = buildExternalHelpers();
+      const script = buildExternalHelpers();
       assert.ok(script.indexOf("classCallCheck") >= -1);
       assert.ok(script.indexOf("inherits") >= 0);
     });
 
     it("whitelist", function () {
-      let script = buildExternalHelpers(["inherits"]);
+      const script = buildExternalHelpers(["inherits"]);
       assert.ok(script.indexOf("classCallCheck") === -1);
       assert.ok(script.indexOf("inherits") >= 0);
     });
 
     it("empty whitelist", function () {
-      let script = buildExternalHelpers([]);
+      const script = buildExternalHelpers([]);
       assert.ok(script.indexOf("classCallCheck") === -1);
       assert.ok(script.indexOf("inherits") === -1);
     });
 
     it("underscored", function () {
-      let script = buildExternalHelpers(["typeof"]);
+      const script = buildExternalHelpers(["typeof"]);
       assert.ok(script.indexOf("typeof") >= 0);
     });
   });
diff --git a/packages/babel-core/test/browserify.js b/packages/babel-core/test/browserify.js
index 9923f6c52ea8..730ace945ab7 100644
--- a/packages/babel-core/test/browserify.js
+++ b/packages/babel-core/test/browserify.js
@@ -1,11 +1,11 @@
-let browserify = require("browserify");
-let assert = require("assert");
-let path = require("path");
-let vm = require("vm");
+const browserify = require("browserify");
+const assert = require("assert");
+const path = require("path");
+const vm = require("vm");
 
 describe("browserify", function() {
   it("babel/register may be used without breaking browserify", function(done) {
-    let bundler = browserify(path.join(__dirname, "fixtures/browserify/register.js"));
+    const bundler = browserify(path.join(__dirname, "fixtures/browserify/register.js"));
 
     bundler.bundle(function(err, bundle) {
       if (err) return done(err);
diff --git a/packages/babel-core/test/config-chain.js b/packages/babel-core/test/config-chain.js
index 0a0b952d5123..95d17eeb15dc 100644
--- a/packages/babel-core/test/config-chain.js
+++ b/packages/babel-core/test/config-chain.js
@@ -1,9 +1,9 @@
-let assert = require("assert");
-let path = require("path");
-let buildConfigChain = require("../lib/transformation/file/options/build-config-chain");
+const assert = require("assert");
+const path = require("path");
+const buildConfigChain = require("../lib/transformation/file/options/build-config-chain");
 
 function fixture() {
-  let args = [__dirname, "fixtures", "config"];
+  const args = [__dirname, "fixtures", "config"];
   for (let i = 0; i < arguments.length; i ++) {
     args.push(arguments[i]);
   }
@@ -28,11 +28,11 @@ describe("buildConfigChain", function () {
   });
 
   it("dir1", function () {
-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
       filename: fixture("dir1", "src.js")
     });
 
-    let expected = [
+    const expected = [
       {
         options: {
           plugins: [
@@ -77,11 +77,11 @@ describe("buildConfigChain", function () {
   });
 
   it("dir2", function () {
-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
       filename: fixture("dir2", "src.js")
     });
 
-    let expected = [
+    const expected = [
       {
         options: {
           plugins: [
@@ -116,11 +116,11 @@ describe("buildConfigChain", function () {
   });
 
   it("env - base", function () {
-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
       filename: fixture("env", "src.js")
     });
 
-    let expected = [
+    const expected = [
       {
         options: {
           plugins: [
@@ -157,11 +157,11 @@ describe("buildConfigChain", function () {
   it("env - foo", function () {
     process.env.NODE_ENV = "foo";
 
-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
       filename: fixture("env", "src.js")
     });
 
-    let expected = [
+    const expected = [
       {
         options: {
           plugins: [
@@ -209,11 +209,11 @@ describe("buildConfigChain", function () {
     process.env.NODE_ENV = "foo"; // overridden
     process.env.NODE_ENV = "bar";
 
-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
       filename: fixture("env", "src.js")
     });
 
-    let expected = [
+    const expected = [
       {
         options: {
           plugins: [
@@ -261,11 +261,11 @@ describe("buildConfigChain", function () {
   it("env - foo", function () {
     process.env.NODE_ENV = "foo";
 
-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
       filename: fixture("pkg", "src.js")
     });
 
-    let expected = [
+    const expected = [
       {
         options: {
           plugins: ["pkg-plugin"]
diff --git a/packages/babel-core/test/evaluation.js b/packages/babel-core/test/evaluation.js
index 1af3bc739933..4378bb417649 100644
--- a/packages/babel-core/test/evaluation.js
+++ b/packages/babel-core/test/evaluation.js
@@ -1,14 +1,14 @@
-let traverse = require("babel-traverse").default;
-let assert   = require("assert");
-let parse    = require("babylon").parse;
+const traverse = require("babel-traverse").default;
+const assert   = require("assert");
+const parse    = require("babylon").parse;
 
 describe("evaluation", function () {
   function addTest(code, type, value, notConfident) {
     it(type + ": " + code, function () {
-      let visitor = {};
+      const visitor = {};
 
       visitor[type] = function (path) {
-        let evaluate = path.evaluate();
+        const evaluate = path.evaluate();
         assert.equal(evaluate.confident, !notConfident);
         assert.deepEqual(evaluate.value, value);
       };
diff --git a/packages/babel-core/test/get-possible-plugin-names.js b/packages/babel-core/test/get-possible-plugin-names.js
index 4a9ec2d08030..d9d730bd6cda 100644
--- a/packages/babel-core/test/get-possible-plugin-names.js
+++ b/packages/babel-core/test/get-possible-plugin-names.js
@@ -1,5 +1,5 @@
-let assert                 = require("assert");
-let getPossiblePluginNames = require("../lib/helpers/get-possible-plugin-names");
+const assert                 = require("assert");
+const getPossiblePluginNames = require("../lib/helpers/get-possible-plugin-names");
 
 describe("getPossiblePluginNames", function () {
   it("adds the babel-plugin prefix", function() {
diff --git a/packages/babel-core/test/get-possible-preset-names.js b/packages/babel-core/test/get-possible-preset-names.js
index 88f9166c8388..64de8897b884 100644
--- a/packages/babel-core/test/get-possible-preset-names.js
+++ b/packages/babel-core/test/get-possible-preset-names.js
@@ -1,5 +1,5 @@
-let assert                 = require("assert");
-let getPossiblePresetNames = require("../lib/helpers/get-possible-preset-names");
+const assert                 = require("assert");
+const getPossiblePresetNames = require("../lib/helpers/get-possible-preset-names");
 
 describe("getPossiblePresetNames", function () {
   it("adds the babel-preset prefix", function() {
diff --git a/packages/babel-core/test/option-manager.js b/packages/babel-core/test/option-manager.js
index 592c431d5481..2606a4bca0d6 100644
--- a/packages/babel-core/test/option-manager.js
+++ b/packages/babel-core/test/option-manager.js
@@ -17,7 +17,7 @@ describe("option-manager", () => {
     it("throws for removed babel 5 options", () => {
       return assert.throws(
         () => {
-          let opt = new OptionManager(new Logger(null, "unknown"));
+          const opt = new OptionManager(new Logger(null, "unknown"));
           opt.init({
             "randomOption": true
           });
@@ -29,7 +29,7 @@ describe("option-manager", () => {
     it("throws for removed babel 5 options", () => {
       return assert.throws(
         () => {
-          let opt = new OptionManager(new Logger(null, "unknown"));
+          const opt = new OptionManager(new Logger(null, "unknown"));
           opt.init({
             "auxiliaryComment": true,
             "blacklist": true
@@ -42,7 +42,7 @@ describe("option-manager", () => {
     it("throws for resolved but erroring preset", () => {
       return assert.throws(
         () => {
-          let opt = new OptionManager(new Logger(null, "unknown"));
+          const opt = new OptionManager(new Logger(null, "unknown"));
           opt.init({
             "presets": [path.join(__dirname, "fixtures/option-manager/not-a-preset")]
           });
@@ -54,7 +54,7 @@ describe("option-manager", () => {
     it("throws for invalid preset configuration", function() {
       return assert.throws(
         function () {
-          let opt = new OptionManager(new Logger(null, "unknown"));
+          const opt = new OptionManager(new Logger(null, "unknown"));
           opt.init({
             "presets": [{ option: "value" }]
           });
@@ -67,8 +67,8 @@ describe("option-manager", () => {
   describe("presets", function () {
     function presetTest(name) {
       it(name, function () {
-        let opt = new OptionManager(new Logger(null, "unknown"));
-        let options = opt.init({
+        const opt = new OptionManager(new Logger(null, "unknown"));
+        const options = opt.init({
           "presets": [path.join(__dirname, "fixtures/option-manager/presets", name)]
         });
 
diff --git a/packages/babel-core/test/path.js b/packages/babel-core/test/path.js
index c76fc31e59bd..e60c68c9f8b6 100644
--- a/packages/babel-core/test/path.js
+++ b/packages/babel-core/test/path.js
@@ -1,12 +1,12 @@
-let transform = require("../lib/api/node").transform;
-let Plugin    = require("../lib/transformation/plugin");
-let chai      = require("chai");
+const transform = require("../lib/api/node").transform;
+const Plugin    = require("../lib/transformation/plugin");
+const chai      = require("chai");
 
 describe("traversal path", function () {
   it("replaceWithSourceString", function () {
-    let expectCode = "function foo() {}";
+    const expectCode = "function foo() {}";
 
-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
       plugins: [new Plugin({
         visitor: {
           FunctionDeclaration: function (path) {
@@ -20,9 +20,9 @@ describe("traversal path", function () {
   });
 
   it("replaceWith (arrow expression body to block statement body)", function () {
-    let expectCode = "var fn = () => true;";
+    const expectCode = "var fn = () => true;";
 
-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
       plugins: [new Plugin({
         visitor: {
           ArrowFunctionExpression: function (path) {
@@ -45,9 +45,9 @@ describe("traversal path", function () {
   });
 
   it("replaceWith (arrow block statement body to expression body)", function () {
-    let expectCode = "var fn = () => { return true; }";
+    const expectCode = "var fn = () => { return true; }";
 
-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
       plugins: [new Plugin({
         visitor: {
           ArrowFunctionExpression: function (path) {
@@ -64,9 +64,9 @@ describe("traversal path", function () {
   });
 
   it("replaceWith (for-in left expression to variable declaration)", function () {
-    let expectCode = "for (KEY in right);";
+    const expectCode = "for (KEY in right);";
 
-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
       plugins: [new Plugin({
         visitor: {
           ForInStatement: function (path) {
@@ -90,9 +90,9 @@ describe("traversal path", function () {
   });
 
   it("replaceWith (for-in left variable declaration to expression)", function () {
-    let expectCode = "for (var KEY in right);";
+    const expectCode = "for (var KEY in right);";
 
-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
       plugins: [new Plugin({
         visitor: {
           ForInStatement: function (path) {
@@ -109,9 +109,9 @@ describe("traversal path", function () {
   });
 
   it("replaceWith (for-loop left expression to variable declaration)", function () {
-    let expectCode = "for (KEY;;);";
+    const expectCode = "for (KEY;;);";
 
-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
       plugins: [new Plugin({
         visitor: {
           ForStatement: function (path) {
@@ -135,9 +135,9 @@ describe("traversal path", function () {
   });
 
   it("replaceWith (for-loop left variable declaration to expression)", function () {
-    let expectCode = "for (var KEY;;);";
+    const expectCode = "for (var KEY;;);";
 
-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
       plugins: [new Plugin({
         visitor: {
           ForStatement: function (path) {
diff --git a/packages/babel-core/test/resolution.js b/packages/babel-core/test/resolution.js
index bdffce466d98..622ddac038c8 100644
--- a/packages/babel-core/test/resolution.js
+++ b/packages/babel-core/test/resolution.js
@@ -1,14 +1,14 @@
-let assert = require("assert");
-let async = require("async");
-let babel = require("../lib/api/node");
-let fs = require("fs");
-let path = require("path");
+const assert = require("assert");
+const async = require("async");
+const babel = require("../lib/api/node");
+const fs = require("fs");
+const path = require("path");
 
 // Test that plugins & presets are resolved relative to `filename`.
 describe("addon resolution", function () {
   it("addon resolution", function (done) {
-    let fixtures = {};
-    let paths = {};
+    const fixtures = {};
+    const paths = {};
 
     paths.fixtures = path.join(
       __dirname,
@@ -33,7 +33,7 @@ describe("addon resolution", function () {
     function fixturesReady (err) {
       if (err) return done(err);
 
-      let actual = babel.transform(fixtures.actual, {
+      const actual = babel.transform(fixtures.actual, {
         filename: paths.actual,
         plugins: ["addons/plugin"],
         presets: ["addons/preset"],
diff --git a/packages/babel-core/test/util.js b/packages/babel-core/test/util.js
index ae1e83e91399..1e879f41a585 100644
--- a/packages/babel-core/test/util.js
+++ b/packages/babel-core/test/util.js
@@ -1,6 +1,6 @@
-let assert = require("assert");
-let util   = require("../lib/util");
-let t      = require("babel-types");
+const assert = require("assert");
+const util   = require("../lib/util");
+const t      = require("babel-types");
 
 describe("util", function () {
   it("canCompile", function () {
@@ -36,7 +36,7 @@ describe("util", function () {
     assert.deepEqual(util.list(["foo", "bar"]), ["foo", "bar"]);
     assert.deepEqual(util.list(/foo/), [/foo/]);
 
-    let date = new Date;
+    const date = new Date;
     assert.deepEqual(util.list(date), [date]);
   });
 
@@ -84,8 +84,8 @@ describe("util", function () {
   });
 
   it("shouldIgnore", function () {
-    let reIgnore = /\-reIgnore\.js/;
-    let fnIgnore = function (src) {
+    const reIgnore = /\-reIgnore\.js/;
+    const fnIgnore = function (src) {
       if (src.indexOf("fnIgnore") > 0) {
         return true;
       }
diff --git a/packages/babel-generator/src/buffer.js b/packages/babel-generator/src/buffer.js
index 69c61db86470..72894c440a25 100644
--- a/packages/babel-generator/src/buffer.js
+++ b/packages/babel-generator/src/buffer.js
@@ -153,7 +153,7 @@ export default class Buffer {
   source(prop: string, loc: Location): void {
     if (prop && !loc) return;
 
-    let pos = loc ? loc[prop] : null;
+    const pos = loc ? loc[prop] : null;
 
     this._sourcePosition.identifierName = loc && loc.identifierName || null;
     this._sourcePosition.line = pos ? pos.line : null;
@@ -169,10 +169,10 @@ export default class Buffer {
     if (!this._map) return cb();
 
     // Use the call stack to manage a stack of "source location" data.
-    let originalLine = this._sourcePosition.line;
-    let originalColumn = this._sourcePosition.column;
-    let originalFilename = this._sourcePosition.filename;
-    let originalIdentifierName = this._sourcePosition.identifierName;
+    const originalLine = this._sourcePosition.line;
+    const originalColumn = this._sourcePosition.column;
+    const originalFilename = this._sourcePosition.filename;
+    const originalIdentifierName = this._sourcePosition.identifierName;
 
     this.source(prop, loc);
 
diff --git a/packages/babel-generator/src/generators/base.js b/packages/babel-generator/src/generators/base.js
index dec9fe11bf3e..ec2dd763641c 100644
--- a/packages/babel-generator/src/generators/base.js
+++ b/packages/babel-generator/src/generators/base.js
@@ -15,7 +15,7 @@ export function BlockStatement(node: Object) {
   this.token("{");
   this.printInnerComments(node);
 
-  let hasDirectives = node.directives && node.directives.length;
+  const hasDirectives = node.directives && node.directives.length;
 
   if (node.body.length || hasDirectives) {
     this.newline();
diff --git a/packages/babel-generator/src/generators/expressions.js b/packages/babel-generator/src/generators/expressions.js
index b38cebed54a6..c4c581eddf16 100644
--- a/packages/babel-generator/src/generators/expressions.js
+++ b/packages/babel-generator/src/generators/expressions.js
@@ -94,7 +94,7 @@ export function CallExpression(node: Object) {
 
   this.token("(");
 
-  let isPrettyCall = node._prettyCall;
+  const isPrettyCall = node._prettyCall;
 
   let separator;
   if (isPrettyCall) {
@@ -127,15 +127,15 @@ function buildYieldAwait(keyword: string) {
 
     if (node.argument) {
       this.space();
-      let terminatorState = this.startTerminatorless();
+      const terminatorState = this.startTerminatorless();
       this.print(node.argument, node);
       this.endTerminatorless(terminatorState);
     }
   };
 }
 
-export let YieldExpression = buildYieldAwait("yield");
-export let AwaitExpression = buildYieldAwait("await");
+export const YieldExpression = buildYieldAwait("yield");
+export const AwaitExpression = buildYieldAwait("await");
 
 export function EmptyStatement() {
   this.semicolon(true /* force */);
@@ -159,7 +159,7 @@ export function AssignmentPattern(node: Object) {
 export function AssignmentExpression(node: Object, parent: Object) {
   // Somewhere inside a for statement `init` node but doesn't usually
   // needs a paren except for `in` expressions: `for (a in b ? a : b;;)`
-  let parens = this.inForStatementInitCounter && node.operator === "in" &&
+  const parens = this.inForStatementInitCounter && node.operator === "in" &&
                !n.needsParens(node, parent);
 
   if (parens) {
diff --git a/packages/babel-generator/src/generators/flow.js b/packages/babel-generator/src/generators/flow.js
index 9ef56bdde494..668ba0e2b3c1 100644
--- a/packages/babel-generator/src/generators/flow.js
+++ b/packages/babel-generator/src/generators/flow.js
@@ -264,7 +264,7 @@ export function ObjectTypeAnnotation(node: Object) {
     this.token("{");
   }
 
-  let props = node.properties.concat(node.callProperties, node.indexers);
+  const props = node.properties.concat(node.callProperties, node.indexers);
 
   if (props.length) {
     this.space();
diff --git a/packages/babel-generator/src/generators/jsx.js b/packages/babel-generator/src/generators/jsx.js
index 7da237ff6a90..521b479ae086 100644
--- a/packages/babel-generator/src/generators/jsx.js
+++ b/packages/babel-generator/src/generators/jsx.js
@@ -47,12 +47,12 @@ export function JSXText(node: Object) {
 }
 
 export function JSXElement(node: Object) {
-  let open = node.openingElement;
+  const open = node.openingElement;
   this.print(open, node);
   if (open.selfClosing) return;
 
   this.indent();
-  for (let child of (node.children: Array<Object>)) {
+  for (const child of (node.children: Array<Object>)) {
     this.print(child, node);
   }
   this.dedent();
diff --git a/packages/babel-generator/src/generators/methods.js b/packages/babel-generator/src/generators/methods.js
index 720aa0c87bd3..97bbcd312976 100644
--- a/packages/babel-generator/src/generators/methods.js
+++ b/packages/babel-generator/src/generators/methods.js
@@ -17,8 +17,8 @@ export function _params(node: Object) {
 }
 
 export function _method(node: Object) {
-  let kind = node.kind;
-  let key  = node.key;
+  const kind = node.kind;
+  const key  = node.key;
 
   if (kind === "method" || kind === "init") {
     if (node.generator) {
diff --git a/packages/babel-generator/src/generators/modules.js b/packages/babel-generator/src/generators/modules.js
index 6a87a0251253..59f19aa8dd85 100644
--- a/packages/babel-generator/src/generators/modules.js
+++ b/packages/babel-generator/src/generators/modules.js
@@ -69,7 +69,7 @@ export function ExportDefaultDeclaration() {
 
 function ExportDeclaration(node: Object) {
   if (node.declaration) {
-    let declar = node.declaration;
+    const declar = node.declaration;
     this.print(declar, node);
     if (!t.isStatement(declar)) this.semicolon();
   } else {
@@ -78,12 +78,12 @@ function ExportDeclaration(node: Object) {
       this.space();
     }
 
-    let specifiers = node.specifiers.slice(0);
+    const specifiers = node.specifiers.slice(0);
 
     // print "special" specifiers first
     let hasSpecial = false;
     while (true) {
-      let first = specifiers[0];
+      const first = specifiers[0];
       if (t.isExportDefaultSpecifier(first) || t.isExportNamespaceSpecifier(first)) {
         hasSpecial = true;
         this.print(specifiers.shift(), node);
@@ -126,11 +126,11 @@ export function ImportDeclaration(node: Object) {
     this.space();
   }
 
-  let specifiers = node.specifiers.slice(0);
+  const specifiers = node.specifiers.slice(0);
   if (specifiers && specifiers.length) {
     // print "special" specifiers first
     while (true) {
-      let first = specifiers[0];
+      const first = specifiers[0];
       if (t.isImportDefaultSpecifier(first) || t.isImportNamespaceSpecifier(first)) {
         this.print(specifiers.shift(), node);
         if (specifiers.length) {
diff --git a/packages/babel-generator/src/generators/statements.js b/packages/babel-generator/src/generators/statements.js
index a5ec4818820f..b9e8e736b344 100644
--- a/packages/babel-generator/src/generators/statements.js
+++ b/packages/babel-generator/src/generators/statements.js
@@ -17,7 +17,7 @@ export function IfStatement(node: Object) {
   this.token(")");
   this.space();
 
-  let needsBlock = node.alternate && t.isIfStatement(getLastStatement(node.consequent));
+  const needsBlock = node.alternate && t.isIfStatement(getLastStatement(node.consequent));
   if (needsBlock) {
     this.token("{");
     this.newline();
@@ -80,7 +80,7 @@ export function WhileStatement(node: Object) {
   this.printBlock(node);
 }
 
-let buildForXStatement = function (op) {
+const buildForXStatement = function (op) {
   return function (node: Object) {
     this.word("for");
     this.space();
@@ -101,9 +101,9 @@ let buildForXStatement = function (op) {
   };
 };
 
-export let ForInStatement = buildForXStatement("in");
-export let ForOfStatement = buildForXStatement("of");
-export let ForAwaitStatement = buildForXStatement("await");
+export const ForInStatement = buildForXStatement("in");
+export const ForOfStatement = buildForXStatement("of");
+export const ForAwaitStatement = buildForXStatement("await");
 
 export function DoWhileStatement(node: Object) {
   this.word("do");
@@ -122,11 +122,11 @@ function buildLabelStatement(prefix, key = "label") {
   return function (node: Object) {
     this.word(prefix);
 
-    let label = node[key];
+    const label = node[key];
     if (label) {
       this.space();
 
-      let terminatorState = this.startTerminatorless();
+      const terminatorState = this.startTerminatorless();
       this.print(label, node);
       this.endTerminatorless(terminatorState);
     }
@@ -135,10 +135,10 @@ function buildLabelStatement(prefix, key = "label") {
   };
 }
 
-export let ContinueStatement = buildLabelStatement("continue");
-export let ReturnStatement   = buildLabelStatement("return", "argument");
-export let BreakStatement    = buildLabelStatement("break");
-export let ThrowStatement    = buildLabelStatement("throw", "argument");
+export const ContinueStatement = buildLabelStatement("continue");
+export const ReturnStatement   = buildLabelStatement("return", "argument");
+export const BreakStatement    = buildLabelStatement("break");
+export const ThrowStatement    = buildLabelStatement("throw", "argument");
 
 export function LabeledStatement(node: Object) {
   this.print(node.label, node);
@@ -242,7 +242,7 @@ export function VariableDeclaration(node: Object, parent: Object) {
   let hasInits = false;
   // don't add whitespace to loop heads
   if (!t.isFor(parent)) {
-    for (let declar of (node.declarations: Array<Object>)) {
+    for (const declar of (node.declarations: Array<Object>)) {
       if (declar.init) {
         // has an init so let's split it up over multiple lines
         hasInits = true;
diff --git a/packages/babel-generator/src/generators/template-literals.js b/packages/babel-generator/src/generators/template-literals.js
index 127b64b0cd96..a49b8615edb9 100644
--- a/packages/babel-generator/src/generators/template-literals.js
+++ b/packages/babel-generator/src/generators/template-literals.js
@@ -7,13 +7,13 @@ export function TemplateElement(node: Object, parent: Object) {
   const isFirst = parent.quasis[0] === node;
   const isLast = parent.quasis[parent.quasis.length - 1] === node;
 
-  let value = (isFirst ? "`" : "}") + node.value.raw + (isLast ? "`" : "${");
+  const value = (isFirst ? "`" : "}") + node.value.raw + (isLast ? "`" : "${");
 
   this.token(value);
 }
 
 export function TemplateLiteral(node: Object) {
-  let quasis = node.quasis;
+  const quasis = node.quasis;
 
   for (let i = 0; i < quasis.length; i++) {
     this.print(quasis[i], node);
diff --git a/packages/babel-generator/src/generators/types.js b/packages/babel-generator/src/generators/types.js
index 3f803c8e11eb..fa8498aff7d1 100644
--- a/packages/babel-generator/src/generators/types.js
+++ b/packages/babel-generator/src/generators/types.js
@@ -32,7 +32,7 @@ export {
 };
 
 export function ObjectExpression(node: Object) {
-  let props = node.properties;
+  const props = node.properties;
 
   this.token("{");
   this.printInnerComments(node);
@@ -84,14 +84,14 @@ export function ObjectProperty(node: Object) {
 }
 
 export function ArrayExpression(node: Object) {
-  let elems = node.elements;
-  let len   = elems.length;
+  const elems = node.elements;
+  const len   = elems.length;
 
   this.token("[");
   this.printInnerComments(node);
 
   for (let i = 0; i < elems.length; i++) {
-    let elem = elems[i];
+    const elem = elems[i];
     if (elem) {
       if (i > 0) this.space();
       this.print(elem, node);
@@ -136,7 +136,7 @@ export function NumericLiteral(node: Object) {
 }
 
 export function StringLiteral(node: Object, parent: Object) {
-  let raw = this.getPossibleRaw(node);
+  const raw = this.getPossibleRaw(node);
   if (!this.format.minified && raw != null) {
     this.token(raw);
     return;
@@ -150,7 +150,7 @@ export function StringLiteral(node: Object, parent: Object) {
   if (this.format.jsonCompatibleStrings) {
     opts.json = true;
   }
-  let val = jsesc(node.value, opts);
+  const val = jsesc(node.value, opts);
 
   return this.token(val);
 }
diff --git a/packages/babel-generator/src/index.js b/packages/babel-generator/src/index.js
index c212cbdf7c05..a3031f16796e 100644
--- a/packages/babel-generator/src/index.js
+++ b/packages/babel-generator/src/index.js
@@ -14,8 +14,8 @@ class Generator extends Printer {
     opts = opts || {};
 
     const tokens = ast.tokens || [];
-    let format = normalizeOptions(code, opts, tokens);
-    let map = opts.sourceMaps ? new SourceMap(opts, code) : null;
+    const format = normalizeOptions(code, opts, tokens);
+    const map = opts.sourceMaps ? new SourceMap(opts, code) : null;
     super(format, map, tokens);
 
     this.ast = ast;
@@ -44,11 +44,11 @@ class Generator extends Printer {
 function normalizeOptions(code, opts, tokens): Format {
   let style = "  ";
   if (code && typeof code === "string") {
-    let indent = detectIndent(code).indent;
+    const indent = detectIndent(code).indent;
     if (indent && indent !== " ") style = indent;
   }
 
-  let format = {
+  const format = {
     auxiliaryCommentBefore: opts.auxiliaryCommentBefore,
     auxiliaryCommentAfter: opts.auxiliaryCommentAfter,
     shouldPrintComment: opts.shouldPrintComment,
@@ -101,7 +101,7 @@ function findCommonStringDelimiter(code, tokens) {
     return DEFAULT_STRING_DELIMITER;
   }
 
-  let occurences = {
+  const occurences = {
     single: 0,
     double: 0
   };
@@ -109,10 +109,10 @@ function findCommonStringDelimiter(code, tokens) {
   let checked = 0;
 
   for (let i = 0; i < tokens.length; i++) {
-    let token = tokens[i];
+    const token = tokens[i];
     if (token.type.label !== "string") continue;
 
-    let raw = code.slice(token.start, token.end);
+    const raw = code.slice(token.start, token.end);
     if (raw[0] === "'") {
       occurences.single++;
     } else {
@@ -145,6 +145,6 @@ export class CodeGenerator {
 }
 
 export default function (ast: Object, opts: Object, code: string): Object {
-  let gen = new Generator(ast, opts, code);
+  const gen = new Generator(ast, opts, code);
   return gen.generate();
 }
diff --git a/packages/babel-generator/src/node/index.js b/packages/babel-generator/src/node/index.js
index 971f478a3da5..c885f8032bd3 100644
--- a/packages/babel-generator/src/node/index.js
+++ b/packages/babel-generator/src/node/index.js
@@ -3,22 +3,22 @@ import * as parens from "./parentheses";
 import * as t from "babel-types";
 
 function expandAliases(obj) {
-  let newObj = {};
+  const newObj = {};
 
   function add(type, func) {
-    let fn = newObj[type];
+    const fn = newObj[type];
     newObj[type] = fn ? function(node, parent, stack) {
-      let result = fn(node, parent, stack);
+      const result = fn(node, parent, stack);
 
       return result == null ? func(node, parent, stack) : result;
     } : func;
   }
 
-  for (let type of Object.keys(obj)) {
+  for (const type of Object.keys(obj)) {
 
-    let aliases = t.FLIPPED_ALIAS_KEYS[type];
+    const aliases = t.FLIPPED_ALIAS_KEYS[type];
     if (aliases) {
-      for (let alias of aliases) {
+      for (const alias of aliases) {
         add(alias, obj[type]);
       }
     } else {
@@ -31,12 +31,12 @@ function expandAliases(obj) {
 
 // Rather than using `t.is` on each object property, we pre-expand any type aliases
 // into concrete types so that the 'find' call below can be as fast as possible.
-let expandedParens = expandAliases(parens);
-let expandedWhitespaceNodes = expandAliases(whitespace.nodes);
-let expandedWhitespaceList = expandAliases(whitespace.list);
+const expandedParens = expandAliases(parens);
+const expandedWhitespaceNodes = expandAliases(whitespace.nodes);
+const expandedWhitespaceList = expandAliases(whitespace.list);
 
 function find(obj, node, parent, printStack) {
-  let fn = obj[node.type];
+  const fn = obj[node.type];
   return fn ? fn(node, parent, printStack) : null;
 }
 
@@ -63,7 +63,7 @@ export function needsWhitespace(node, parent, type) {
   let linesInfo = find(expandedWhitespaceNodes, node, parent);
 
   if (!linesInfo) {
-    let items = find(expandedWhitespaceList, node, parent);
+    const items = find(expandedWhitespaceList, node, parent);
     if (items) {
       for (let i = 0; i < items.length; i++) {
         linesInfo = needsWhitespace(items[i], node, type);
diff --git a/packages/babel-generator/src/node/parentheses.js b/packages/babel-generator/src/node/parentheses.js
index 199252d96e90..203ed8a3aa76 100644
--- a/packages/babel-generator/src/node/parentheses.js
+++ b/packages/babel-generator/src/node/parentheses.js
@@ -60,11 +60,11 @@ export function Binary(node: Object, parent: Object): boolean {
   }
 
   if (t.isBinary(parent)) {
-    let parentOp  = parent.operator;
-    let parentPos = PRECEDENCE[parentOp];
+    const parentOp  = parent.operator;
+    const parentPos = PRECEDENCE[parentOp];
 
-    let nodeOp = node.operator;
-    let nodePos = PRECEDENCE[nodeOp];
+    const nodeOp = node.operator;
+    const nodePos = PRECEDENCE[nodeOp];
 
     if (parentPos > nodePos) {
       return true;
diff --git a/packages/babel-generator/src/node/whitespace.js b/packages/babel-generator/src/node/whitespace.js
index 6ba8522e083b..21320e33d309 100644
--- a/packages/babel-generator/src/node/whitespace.js
+++ b/packages/babel-generator/src/node/whitespace.js
@@ -69,7 +69,7 @@ exports.nodes = {
    */
 
   AssignmentExpression(node: Object): ?WhitespaceObject {
-    let state = crawl(node.right);
+    const state = crawl(node.right);
     if ((state.hasCall && state.hasHelper) || state.hasFunction) {
       return {
         before: state.hasFunction,
@@ -131,11 +131,11 @@ exports.nodes = {
 
   VariableDeclaration(node: Object): ?WhitespaceObject {
     for (let i = 0; i < node.declarations.length; i++) {
-      let declar = node.declarations[i];
+      const declar = node.declarations[i];
 
       let enabled = isHelper(declar.id) && !isType(declar.init);
       if (!enabled) {
-        let state = crawl(declar.init);
+        const state = crawl(declar.init);
         enabled = (isHelper(declar.init) && state.hasCall) || state.hasFunction;
       }
 
diff --git a/packages/babel-generator/src/printer.js b/packages/babel-generator/src/printer.js
index a9af0094297b..e03298d1f909 100644
--- a/packages/babel-generator/src/printer.js
+++ b/packages/babel-generator/src/printer.js
@@ -238,7 +238,7 @@ export default class Printer {
 
   _maybeAddParen(str: string): void {
     // see startTerminatorless() instance method
-    let parenPushNewlineState = this._parenPushNewlineState;
+    const parenPushNewlineState = this._parenPushNewlineState;
     if (!parenPushNewlineState) return;
     this._parenPushNewlineState = null;
 
@@ -314,19 +314,19 @@ export default class Printer {
   print(node, parent) {
     if (!node) return;
 
-    let oldConcise = this.format.concise;
+    const oldConcise = this.format.concise;
     if (node._compact) {
       this.format.concise = true;
     }
 
-    let printMethod = this[node.type];
+    const printMethod = this[node.type];
     if (!printMethod) {
       throw new ReferenceError(`unknown node of type ${JSON.stringify(node.type)} with constructor ${JSON.stringify(node && node.constructor.name)}`);
     }
 
     this._printStack.push(node);
 
-    let oldInAux = this._insideAux;
+    const oldInAux = this._insideAux;
     this._insideAux = !node.loc;
     this._maybeAddAuxComment(this._insideAux && !oldInAux);
 
@@ -340,7 +340,7 @@ export default class Printer {
 
     this._printLeadingComments(node, parent);
 
-    let loc = (t.isProgram(node) || t.isFile(node)) ? null : node.loc;
+    const loc = (t.isProgram(node) || t.isFile(node)) ? null : node.loc;
     this.withSource("start", loc, () => {
       this[node.type](node, parent);
     });
@@ -388,7 +388,7 @@ export default class Printer {
   }
 
   getPossibleRaw(node) {
-    let extra = node.extra;
+    const extra = node.extra;
     if (extra && extra.raw != null && extra.rawValue != null && node.value === extra.rawValue) {
       return extra.raw;
     }
@@ -426,14 +426,14 @@ export default class Printer {
   }
 
   printAndIndentOnComments(node, parent) {
-    let indent = !!node.leadingComments;
+    const indent = !!node.leadingComments;
     if (indent) this.indent();
     this.print(node, parent);
     if (indent) this.dedent();
   }
 
   printBlock(parent) {
-    let node = parent.body;
+    const node = parent.body;
 
     if (!t.isEmptyStatement(node)) {
       this.space();
@@ -544,13 +544,13 @@ export default class Printer {
 
     //
     if (comment.type === "CommentBlock" && this.format.indent.adjustMultilineComment) {
-      let offset = comment.loc && comment.loc.start.column;
+      const offset = comment.loc && comment.loc.start.column;
       if (offset) {
-        let newlineRegex = new RegExp("\\n\\s{1," + offset + "}", "g");
+        const newlineRegex = new RegExp("\\n\\s{1," + offset + "}", "g");
         val = val.replace(newlineRegex, "\n");
       }
 
-      let indentSize = Math.max(this._getIndent().length, this._buf.getCurrentColumn());
+      const indentSize = Math.max(this._getIndent().length, this._buf.getCurrentColumn());
       val = val.replace(/\n(?!$)/g, `\n${repeat(" ", indentSize)}`);
     }
 
@@ -567,7 +567,7 @@ export default class Printer {
   _printComments(comments?: Array<Object>) {
     if (!comments || !comments.length) return;
 
-    for (let comment of comments) {
+    for (const comment of comments) {
       this._printComment(comment);
     }
   }
@@ -578,7 +578,7 @@ function commaSeparator() {
   this.space();
 }
 
-for (let generator of [
+for (const generator of [
   require("./generators/template-literals"),
   require("./generators/expressions"),
   require("./generators/statements"),
diff --git a/packages/babel-generator/src/whitespace.js b/packages/babel-generator/src/whitespace.js
index 2eb4f9b308e3..6a055e02e265 100644
--- a/packages/babel-generator/src/whitespace.js
+++ b/packages/babel-generator/src/whitespace.js
@@ -15,7 +15,7 @@ export default class Whitespace {
   getNewlinesBefore(node) {
     let startToken;
     let endToken;
-    let tokens = this.tokens;
+    const tokens = this.tokens;
 
     let index = this._findToken((token) => token.start - node.start, 0, tokens.length);
     if (index >= 0) {
@@ -34,7 +34,7 @@ export default class Whitespace {
   getNewlinesAfter(node) {
     let startToken;
     let endToken;
-    let tokens = this.tokens;
+    const tokens = this.tokens;
 
     let index = this._findToken((token) => token.end - node.end, 0, tokens.length);
     if (index >= 0) {
@@ -58,8 +58,8 @@ export default class Whitespace {
   _getNewlinesBetween(startToken, endToken) {
     if (!endToken || !endToken.loc) return 0;
 
-    let start = startToken ? startToken.loc.end.line : 1;
-    let end   = endToken.loc.start.line;
+    const start = startToken ? startToken.loc.end.line : 1;
+    const end   = endToken.loc.start.line;
     let lines = 0;
 
     for (let line = start; line < end; line++) {
diff --git a/packages/babel-generator/test/index.js b/packages/babel-generator/test/index.js
index 40d8402d347e..cd84698ba57d 100644
--- a/packages/babel-generator/test/index.js
+++ b/packages/babel-generator/test/index.js
@@ -1,13 +1,13 @@
-let Whitespace = require("../lib/whitespace");
-let Printer    = require("../lib/printer");
-let generate   = require("../lib");
-let assert     = require("assert");
-let parse      = require("babylon").parse;
-let chai       = require("chai");
-let t          = require("babel-types");
-let _          = require("lodash");
-let fs         = require("fs");
-let path       = require("path");
+const Whitespace = require("../lib/whitespace");
+const Printer    = require("../lib/printer");
+const generate   = require("../lib");
+const assert     = require("assert");
+const parse      = require("babylon").parse;
+const chai       = require("chai");
+const t          = require("babel-types");
+const _          = require("lodash");
+const fs         = require("fs");
+const path       = require("path");
 
 describe("generation", function () {
   it("completeness", function () {
@@ -22,16 +22,16 @@ describe("generation", function () {
   });
 
   it("multiple sources", function () {
-    let sources = {
+    const sources = {
       "a.js": "function hi (msg) { console.log(msg); }\n",
       "b.js": "hi('hello');\n"
     };
-    let parsed = _.keys(sources).reduce(function (_parsed, filename) {
+    const parsed = _.keys(sources).reduce(function (_parsed, filename) {
       _parsed[filename] = parse(sources[filename], { sourceFilename: filename });
       return _parsed;
     }, {});
 
-    let combinedAst = {
+    const combinedAst = {
       "type": "File",
       "program": {
         "type": "Program",
@@ -40,7 +40,7 @@ describe("generation", function () {
       }
     };
 
-    let generated = generate.default(combinedAst, { sourceMaps: true }, sources);
+    const generated = generate.default(combinedAst, { sourceMaps: true }, sources);
 
     chai.expect(generated.map).to.deep.equal({
       version: 3,
@@ -128,20 +128,20 @@ describe("generation", function () {
   });
 
   it("identifierName", function () {
-    let code = "function foo() { bar; }\n";
+    const code = "function foo() { bar; }\n";
 
-    let ast = parse(code, { filename: "inline" }).program;
-    let fn = ast.body[0];
+    const ast = parse(code, { filename: "inline" }).program;
+    const fn = ast.body[0];
 
-    let id = fn.id;
+    const id = fn.id;
     id.name += "2";
     id.loc.identifierName = "foo";
 
-    let id2 = fn.body.body[0].expression;
+    const id2 = fn.body.body[0].expression;
     id2.name += "2";
     id2.loc.identiferName = "bar";
 
-    let generated = generate.default(ast, {
+    const generated = generate.default(ast, {
       filename: "inline",
       sourceFileName: "inline",
       sourceMaps: true
@@ -189,10 +189,10 @@ describe("generation", function () {
   });
 
   it("lazy source map generation", function() {
-    let code = "function hi (msg) { console.log(msg); }\n";
+    const code = "function hi (msg) { console.log(msg); }\n";
 
-    let ast = parse(code, { filename: "a.js" }).program;
-    let generated = generate.default(ast, {
+    const ast = parse(code, { filename: "a.js" }).program;
+    const generated = generate.default(ast, {
       sourceFileName: "a.js",
       sourceMaps: true,
     });
@@ -209,12 +209,12 @@ describe("generation", function () {
 describe("programmatic generation", function() {
   it("numeric member expression", function() {
     // Should not generate `0.foo`
-    let mem = t.memberExpression(t.numericLiteral(60702), t.identifier("foo"));
+    const mem = t.memberExpression(t.numericLiteral(60702), t.identifier("foo"));
     new Function(generate.default(mem).code);
   });
 
   it("nested if statements needs block", function() {
-    let ifStatement = t.ifStatement(
+    const ifStatement = t.ifStatement(
       t.stringLiteral("top cond"),
       t.whileStatement(
         t.stringLiteral("while cond"),
@@ -226,17 +226,17 @@ describe("programmatic generation", function() {
       t.expressionStatement(t.stringLiteral("alt"))
     );
 
-    let ast = parse(generate.default(ifStatement).code);
+    const ast = parse(generate.default(ifStatement).code);
     assert.equal(ast.program.body[0].consequent.type, "BlockStatement");
   });
 
   it("prints directives in block with empty body", function() {
-    let blockStatement = t.blockStatement(
+    const blockStatement = t.blockStatement(
       [],
       [t.directive(t.directiveLiteral("use strict"))]
     );
 
-    let output = generate.default(blockStatement).code;
+    const output = generate.default(blockStatement).code;
     assert.equal(output, [
       "{",
       "  \"use strict\";",
@@ -245,7 +245,7 @@ describe("programmatic generation", function() {
   });
 
   it("flow object indentation", function() {
-    let objectStatement = t.objectTypeAnnotation(
+    const objectStatement = t.objectTypeAnnotation(
       [
         t.objectTypeProperty(
           t.identifier("bar"),
@@ -256,7 +256,7 @@ describe("programmatic generation", function() {
       null
     );
 
-    let output = generate.default(objectStatement).code;
+    const output = generate.default(objectStatement).code;
     assert.equal(output, [
       "{",
       "  bar: string;",
@@ -265,7 +265,7 @@ describe("programmatic generation", function() {
   });
 
   it("flow object indentation with empty leading ObjectTypeProperty", function() {
-    let objectStatement = t.objectTypeAnnotation(
+    const objectStatement = t.objectTypeAnnotation(
       [],
       [
         t.objectTypeIndexer(
@@ -276,7 +276,7 @@ describe("programmatic generation", function() {
       ]
     );
 
-    let output = generate.default(objectStatement).code;
+    const output = generate.default(objectStatement).code;
 
     assert.equal(output, [
       "{",
@@ -288,29 +288,29 @@ describe("programmatic generation", function() {
 
 describe("whitespace", function () {
   it("empty token list", function () {
-    let w = new Whitespace([]);
+    const w = new Whitespace([]);
     assert.equal(w.getNewlinesBefore(t.stringLiteral("1")), 0);
   });
 });
 
-let suites = require("babel-helper-fixtures").default(__dirname + "/fixtures");
+const suites = require("babel-helper-fixtures").default(__dirname + "/fixtures");
 
 suites.forEach(function (testSuite) {
   describe("generation/" + testSuite.title, function () {
     _.each(testSuite.tests, function (task) {
       it(task.title, !task.disabled && function () {
-        let expect = task.expect;
-        let actual = task.actual;
-        let actualCode = actual.code;
+        const expect = task.expect;
+        const actual = task.actual;
+        const actualCode = actual.code;
 
         if (actualCode) {
-          let actualAst = parse(actualCode, {
+          const actualAst = parse(actualCode, {
             filename: actual.loc,
             plugins: ["*"],
             strictMode: false,
             sourceType: "module",
           });
-          let result = generate.default(actualAst, task.options, actualCode);
+          const result = generate.default(actualAst, task.options, actualCode);
 
           if (!expect.code && result.code && fs.statSync(path.dirname(expect.loc)).isDirectory() && !process.env.CI) {
             console.log(`New test file created: ${expect.loc}`);
diff --git a/packages/babel-helper-bindify-decorators/src/index.js b/packages/babel-helper-bindify-decorators/src/index.js
index 750c00f56fdf..debc145a4f04 100644
--- a/packages/babel-helper-bindify-decorators/src/index.js
+++ b/packages/babel-helper-bindify-decorators/src/index.js
@@ -2,15 +2,15 @@ import type { NodePath } from "babel-traverse";
 import * as t from "babel-types";
 
 export default function bindifyDecorators(decorators: Array<NodePath>): Array<NodePath> {
-  for (let decoratorPath of decorators) {
-    let decorator = decoratorPath.node;
-    let expression = decorator.expression;
+  for (const decoratorPath of decorators) {
+    const decorator = decoratorPath.node;
+    const expression = decorator.expression;
     if (!t.isMemberExpression(expression)) continue;
 
-    let temp = decoratorPath.scope.maybeGenerateMemoised(expression.object);
+    const temp = decoratorPath.scope.maybeGenerateMemoised(expression.object);
     let ref;
 
-    let nodes = [];
+    const nodes = [];
 
     if (temp) {
       ref = temp;
diff --git a/packages/babel-helper-builder-binary-assignment-operator-visitor/src/index.js b/packages/babel-helper-builder-binary-assignment-operator-visitor/src/index.js
index 0c07eecf9ff2..b3e41d9f7ed3 100644
--- a/packages/babel-helper-builder-binary-assignment-operator-visitor/src/index.js
+++ b/packages/babel-helper-builder-binary-assignment-operator-visitor/src/index.js
@@ -5,7 +5,7 @@ export default function (opts: {
   build: Function;
   operator: string;
 }): Object {
-  let visitor = {};
+  const visitor = {};
 
   function isAssignment(node) {
     return node && node.operator === opts.operator + "=";
@@ -19,11 +19,11 @@ export default function (opts: {
     // hit the `AssignmentExpression` one below
     if (path.isCompletionRecord()) return;
 
-    let expr = path.node.expression;
+    const expr = path.node.expression;
     if (!isAssignment(expr)) return;
 
-    let nodes    = [];
-    let exploded = explode(expr.left, nodes, file, path.scope, true);
+    const nodes    = [];
+    const exploded = explode(expr.left, nodes, file, path.scope, true);
 
     nodes.push(t.expressionStatement(
       buildAssignment(exploded.ref, opts.build(exploded.uid, expr.right))
@@ -33,17 +33,17 @@ export default function (opts: {
   };
 
   visitor.AssignmentExpression = function (path, file) {
-    let { node, scope } = path;
+    const { node, scope } = path;
     if (!isAssignment(node)) return;
 
-    let nodes    = [];
-    let exploded = explode(node.left, nodes, file, scope);
+    const nodes    = [];
+    const exploded = explode(node.left, nodes, file, scope);
     nodes.push(buildAssignment(exploded.ref, opts.build(exploded.uid, node.right)));
     path.replaceWithMultiple(nodes);
   };
 
   visitor.BinaryExpression = function (path) {
-    let { node } = path;
+    const { node } = path;
     if (node.operator === opts.operator) {
       path.replaceWith(opts.build(node.left, node.right));
     }
diff --git a/packages/babel-helper-builder-conditional-assignment-operator-visitor/src/index.js b/packages/babel-helper-builder-conditional-assignment-operator-visitor/src/index.js
index bf8f9d48b952..335f2e50fa91 100644
--- a/packages/babel-helper-builder-conditional-assignment-operator-visitor/src/index.js
+++ b/packages/babel-helper-builder-conditional-assignment-operator-visitor/src/index.js
@@ -8,7 +8,7 @@ export default function (
     is: Function;
   },
 ) {
-  let buildAssignment = function (left, right) {
+  const buildAssignment = function (left, right) {
     return t.assignmentExpression("=", left, right);
   };
 
@@ -16,12 +16,12 @@ export default function (
     // hit the `AssignmentExpression` one below
     if (path.isCompletionRecord()) return;
 
-    let expr = path.node.expression;
+    const expr = path.node.expression;
     if (!opts.is(expr, file)) return;
 
-    let nodes = [];
+    const nodes = [];
 
-    let exploded = explode(expr.left, nodes, file, path.scope);
+    const exploded = explode(expr.left, nodes, file, path.scope);
 
     nodes.push(t.ifStatement(
       opts.build(exploded.uid, file),
@@ -32,11 +32,11 @@ export default function (
   };
 
   exports.AssignmentExpression = function (path, file) {
-    let node = path.node;
+    const node = path.node;
     if (!opts.is(node, file)) return;
 
-    let nodes    = [];
-    let exploded = explode(node.left, nodes, file, path.scope);
+    const nodes    = [];
+    const exploded = explode(node.left, nodes, file, path.scope);
 
     nodes.push(t.logicalExpression(
       "&&",
diff --git a/packages/babel-helper-builder-react-jsx/src/index.js b/packages/babel-helper-builder-react-jsx/src/index.js
index 2438070df0d7..1a0a1ca62632 100644
--- a/packages/babel-helper-builder-react-jsx/src/index.js
+++ b/packages/babel-helper-builder-react-jsx/src/index.js
@@ -11,7 +11,7 @@ type ElementState = {
 };
 
 export default function (opts) {
-  let visitor = {};
+  const visitor = {};
 
   visitor.JSXNamespacedName = function (path) {
     throw path.buildCodeFrameError("Namespace tags are not supported. ReactJSX is not XML.");
@@ -19,7 +19,7 @@ export default function (opts) {
 
   visitor.JSXElement = {
     exit(path, file) {
-      let callExpr = buildElementCall(path.get("openingElement"), file);
+      const callExpr = buildElementCall(path.get("openingElement"), file);
 
       callExpr.arguments = callExpr.arguments.concat(path.node.children);
 
@@ -61,7 +61,7 @@ export default function (opts) {
   }
 
   function convertAttribute(node) {
-    let value = convertAttributeValue(node.value || t.booleanLiteral(true));
+    const value = convertAttributeValue(node.value || t.booleanLiteral(true));
 
     if (t.isStringLiteral(value) && !t.isJSXExpressionContainer(node.value)) {
       value.value = value.value.replace(/\n\s+/g, " ");
@@ -79,8 +79,8 @@ export default function (opts) {
   function buildElementCall(path, file) {
     path.parent.children = t.react.buildChildren(path.parent);
 
-    let tagExpr = convertJSXIdentifier(path.node.name, path.node);
-    let args = [];
+    const tagExpr = convertJSXIdentifier(path.node.name, path.node);
+    const args = [];
 
     let tagName;
     if (t.isIdentifier(tagExpr)) {
@@ -89,7 +89,7 @@ export default function (opts) {
       tagName = tagExpr.value;
     }
 
-    let state: ElementState = {
+    const state: ElementState = {
       tagExpr: tagExpr,
       tagName: tagName,
       args:    args
@@ -124,9 +124,9 @@ export default function (opts) {
 
   function buildOpeningElementAttributes(attribs, file) {
     let _props = [];
-    let objs = [];
+    const objs = [];
 
-    let useBuiltIns = file.opts.useBuiltIns || false;
+    const useBuiltIns = file.opts.useBuiltIns || false;
     if (typeof useBuiltIns !== "boolean") {
       throw new Error("transform-react-jsx currently only accepts a boolean option for useBuiltIns (defaults to false)");
     }
@@ -139,7 +139,7 @@ export default function (opts) {
     }
 
     while (attribs.length) {
-      let prop = attribs.shift();
+      const prop = attribs.shift();
       if (t.isJSXSpreadAttribute(prop)) {
         pushProps();
         objs.push(prop.argument);
diff --git a/packages/babel-helper-call-delegate/src/index.js b/packages/babel-helper-call-delegate/src/index.js
index 1319a662fccc..653a48b172f5 100644
--- a/packages/babel-helper-call-delegate/src/index.js
+++ b/packages/babel-helper-call-delegate/src/index.js
@@ -2,7 +2,7 @@ import hoistVariables from "babel-helper-hoist-variables";
 import type { NodePath } from "babel-traverse";
 import * as t from "babel-types";
 
-let visitor = {
+const visitor = {
   enter(path, state) {
     if (path.isThisExpression()) {
       state.foundThis = true;
@@ -19,8 +19,8 @@ let visitor = {
 };
 
 export default function (path: NodePath, scope = path.scope) {
-  let { node } = path;
-  let container = t.functionExpression(null, [], node.body, node.generator, node.async);
+  const { node } = path;
+  const container = t.functionExpression(null, [], node.body, node.generator, node.async);
 
   let callee = container;
   let args   = [];
@@ -28,7 +28,7 @@ export default function (path: NodePath, scope = path.scope) {
   // todo: only hoist if necessary
   hoistVariables(path, (id) => scope.push({ id }));
 
-  let state = {
+  const state = {
     foundThis: false,
     foundArguments: false
   };
diff --git a/packages/babel-helper-define-map/src/index.js b/packages/babel-helper-define-map/src/index.js
index 653674755507..09de4f9b5821 100644
--- a/packages/babel-helper-define-map/src/index.js
+++ b/packages/babel-helper-define-map/src/index.js
@@ -16,7 +16,7 @@ function toKind(node: Object) {
 }
 
 export function push(mutatorMap: Object, node: Object, kind: string, file, scope?): Object {
-  let alias = t.toKeyAlias(node);
+  const alias = t.toKeyAlias(node);
 
   //
 
@@ -36,7 +36,7 @@ export function push(mutatorMap: Object, node: Object, kind: string, file, scope
   }
 
   if (node.decorators) {
-    let decorators = map.decorators = map.decorators || t.arrayExpression([]);
+    const decorators = map.decorators = map.decorators || t.arrayExpression([]);
     decorators.elements = decorators.elements.concat(node.decorators.map((dec) => dec.expression).reverse());
   }
 
@@ -58,7 +58,7 @@ export function push(mutatorMap: Object, node: Object, kind: string, file, scope
     value.returnType = node.returnType;
   }
 
-  let inheritedKind = toKind(node);
+  const inheritedKind = toKind(node);
   if (!kind || inheritedKind !== "value") {
     kind = inheritedKind;
   }
@@ -77,7 +77,7 @@ export function push(mutatorMap: Object, node: Object, kind: string, file, scope
 }
 
 export function hasComputed(mutatorMap: Object): boolean {
-  for (let key in mutatorMap) {
+  for (const key in mutatorMap) {
     if (mutatorMap[key]._computed) {
       return true;
     }
@@ -86,11 +86,11 @@ export function hasComputed(mutatorMap: Object): boolean {
 }
 
 export function toComputedObjectFromClass(obj: Object): Object {
-  let objExpr = t.arrayExpression([]);
+  const objExpr = t.arrayExpression([]);
 
   for (let i = 0; i < obj.properties.length; i++) {
-    let prop = obj.properties[i];
-    let val = prop.value;
+    const prop = obj.properties[i];
+    const val = prop.value;
     val.properties.unshift(t.objectProperty(t.identifier("key"), t.toComputedKey(prop)));
     objExpr.elements.push(val);
   }
@@ -99,20 +99,20 @@ export function toComputedObjectFromClass(obj: Object): Object {
 }
 
 export function toClassObject(mutatorMap: Object): Object {
-  let objExpr = t.objectExpression([]);
+  const objExpr = t.objectExpression([]);
 
   each(mutatorMap, function (map) {
-    let mapNode = t.objectExpression([]);
+    const mapNode = t.objectExpression([]);
 
-    let propNode = t.objectProperty(map._key, mapNode, map._computed);
+    const propNode = t.objectProperty(map._key, mapNode, map._computed);
 
     each(map, function (node, key) {
       if (key[0] === "_") return;
 
-      let inheritNode = node;
+      const inheritNode = node;
       if (t.isClassMethod(node) || t.isClassProperty(node)) node = node.value;
 
-      let prop = t.objectProperty(t.identifier(key), node);
+      const prop = t.objectProperty(t.identifier(key), node);
       t.inheritsComments(prop, inheritNode);
       t.removeComments(inheritNode);
 
diff --git a/packages/babel-helper-explode-assignable-expression/src/index.js b/packages/babel-helper-explode-assignable-expression/src/index.js
index 8ffdd95fa201..04052f5e3403 100644
--- a/packages/babel-helper-explode-assignable-expression/src/index.js
+++ b/packages/babel-helper-explode-assignable-expression/src/index.js
@@ -31,7 +31,7 @@ function getObjRef(node, nodes, file, scope) {
     throw new Error(`We can't explode this node type ${node.type}`);
   }
 
-  let temp = scope.generateUidIdentifierBasedOnNode(ref);
+  const temp = scope.generateUidIdentifierBasedOnNode(ref);
   nodes.push(t.variableDeclaration("var", [
     t.variableDeclarator(temp, ref)
   ]));
@@ -39,11 +39,11 @@ function getObjRef(node, nodes, file, scope) {
 }
 
 function getPropRef(node, nodes, file, scope) {
-  let prop = node.property;
-  let key = t.toComputedKey(node, prop);
+  const prop = node.property;
+  const key = t.toComputedKey(node, prop);
   if (t.isLiteral(key) && t.isPureish(key)) return key;
 
-  let temp = scope.generateUidIdentifierBasedOnNode(prop);
+  const temp = scope.generateUidIdentifierBasedOnNode(prop);
   nodes.push(t.variableDeclaration("var", [
     t.variableDeclarator(temp, prop)
   ]));
@@ -73,8 +73,8 @@ export default function (
     ref = node;
     uid = obj;
   } else {
-    let prop = getPropRef(node, nodes, file, scope);
-    let computed = node.computed || t.isLiteral(prop);
+    const prop = getPropRef(node, nodes, file, scope);
+    const computed = node.computed || t.isLiteral(prop);
     uid = ref = t.memberExpression(obj, prop, computed);
   }
 
diff --git a/packages/babel-helper-explode-class/src/index.js b/packages/babel-helper-explode-class/src/index.js
index 1091e1ba0638..2ced5be5b179 100644
--- a/packages/babel-helper-explode-class/src/index.js
+++ b/packages/babel-helper-explode-class/src/index.js
@@ -5,12 +5,12 @@ import * as t from "babel-types";
 export default function (classPath) {
   classPath.assertClass();
 
-  let memoisedExpressions = [];
+  const memoisedExpressions = [];
 
   function maybeMemoise(path) {
     if (!path.node || path.isPure()) return;
 
-    let uid = classPath.scope.generateDeclaredUidIdentifier();
+    const uid = classPath.scope.generateDeclaredUidIdentifier();
     memoisedExpressions.push(t.assignmentExpression("=", uid, path.node));
     path.replaceWith(uid);
   }
@@ -24,7 +24,7 @@ export default function (classPath) {
     // bind decorators if they're member expressions
     bindifyDecorators(paths);
 
-    for (let path of paths) {
+    for (const path of paths) {
       maybeMemoise(path);
     }
   }
@@ -32,8 +32,8 @@ export default function (classPath) {
   maybeMemoise(classPath.get("superClass"));
   memoiseDecorators(classPath.get("decorators"), true);
 
-  let methods: Array<NodePath> = classPath.get("body.body");
-  for (let methodPath of methods) {
+  const methods: Array<NodePath> = classPath.get("body.body");
+  for (const methodPath of methods) {
     if (methodPath.is("computed")) {
       maybeMemoise(methodPath.get("key"));
     }
diff --git a/packages/babel-helper-fixtures/src/index.js b/packages/babel-helper-fixtures/src/index.js
index 40faaf153037..e7fcebf1d95e 100644
--- a/packages/babel-helper-fixtures/src/index.js
+++ b/packages/babel-helper-fixtures/src/index.js
@@ -41,23 +41,23 @@ function shouldIgnore(name, blacklist?: Array<string>) {
     return true;
   }
 
-  let ext = path.extname(name);
-  let base = path.basename(name, ext);
+  const ext = path.extname(name);
+  const base = path.basename(name, ext);
 
   return name[0] === "." || ext === ".md" || base === "LICENSE" || base === "options";
 }
 
 export default function get(entryLoc): Array<Suite> {
-  let suites = [];
+  const suites = [];
 
   let rootOpts = {};
-  let rootOptsLoc = resolve(entryLoc + "/options");
+  const rootOptsLoc = resolve(entryLoc + "/options");
   if (rootOptsLoc) rootOpts = require(rootOptsLoc);
 
-  for (let suiteName of fs.readdirSync(entryLoc)) {
+  for (const suiteName of fs.readdirSync(entryLoc)) {
     if (shouldIgnore(suiteName)) continue;
 
-    let suite = {
+    const suite = {
       options: _.clone(rootOpts),
       tests: [],
       title: humanize(suiteName),
@@ -67,24 +67,24 @@ export default function get(entryLoc): Array<Suite> {
     assertDirectory(suite.filename);
     suites.push(suite);
 
-    let suiteOptsLoc = resolve(suite.filename + "/options");
+    const suiteOptsLoc = resolve(suite.filename + "/options");
     if (suiteOptsLoc) suite.options = require(suiteOptsLoc);
 
-    for (let taskName of fs.readdirSync(suite.filename)) {
+    for (const taskName of fs.readdirSync(suite.filename)) {
       push(taskName, suite.filename + "/" + taskName);
     }
 
     function push(taskName, taskDir) {
-      let actualLocAlias = suiteName + "/" + taskName + "/actual.js";
+      const actualLocAlias = suiteName + "/" + taskName + "/actual.js";
       let expectLocAlias = suiteName + "/" + taskName + "/expected.js";
-      let execLocAlias   = suiteName + "/" + taskName + "/exec.js";
+      const execLocAlias   = suiteName + "/" + taskName + "/exec.js";
 
-      let actualLoc = taskDir + "/actual.js";
+      const actualLoc = taskDir + "/actual.js";
       let expectLoc = taskDir + "/expected.js";
       let execLoc   = taskDir + "/exec.js";
 
       if (fs.statSync(taskDir).isFile()) {
-        let ext = path.extname(taskDir);
+        const ext = path.extname(taskDir);
         if (ext !== ".js" && ext !== ".module.js") return;
 
         execLoc = taskDir;
@@ -95,12 +95,12 @@ export default function get(entryLoc): Array<Suite> {
         expectLocAlias += "on";
       }
 
-      let taskOpts = _.cloneDeep(suite.options);
+      const taskOpts = _.cloneDeep(suite.options);
 
-      let taskOptsLoc = resolve(taskDir + "/options");
+      const taskOptsLoc = resolve(taskDir + "/options");
       if (taskOptsLoc) _.merge(taskOpts, require(taskOptsLoc));
 
-      let test = {
+      const test = {
         optionsDir: taskOptsLoc ? path.dirname(taskOptsLoc) : null,
         title: humanize(taskName, true),
         disabled: taskName[0] === ".",
@@ -130,12 +130,12 @@ export default function get(entryLoc): Array<Suite> {
 
       suite.tests.push(test);
 
-      let sourceMappingsLoc = taskDir + "/source-mappings.json";
+      const sourceMappingsLoc = taskDir + "/source-mappings.json";
       if (fs.existsSync(sourceMappingsLoc)) {
         test.sourceMappings = JSON.parse(readFile(sourceMappingsLoc));
       }
 
-      let sourceMapLoc = taskDir + "/source-map.json";
+      const sourceMapLoc = taskDir + "/source-map.json";
       if (fs.existsSync(sourceMapLoc)) {
         test.sourceMap = JSON.parse(readFile(sourceMapLoc));
       }
@@ -146,12 +146,12 @@ export default function get(entryLoc): Array<Suite> {
 }
 
 export function multiple(entryLoc, ignore?: Array<string>) {
-  let categories = {};
+  const categories = {};
 
-  for (let name of fs.readdirSync(entryLoc)) {
+  for (const name of fs.readdirSync(entryLoc)) {
     if (shouldIgnore(name, ignore)) continue;
 
-    let loc = path.join(entryLoc, name);
+    const loc = path.join(entryLoc, name);
     assertDirectory(loc);
 
     categories[name] = get(loc);
diff --git a/packages/babel-helper-function-name/src/index.js b/packages/babel-helper-function-name/src/index.js
index 1e6d8add9d31..7623bf0a10b0 100644
--- a/packages/babel-helper-function-name/src/index.js
+++ b/packages/babel-helper-function-name/src/index.js
@@ -4,7 +4,7 @@ import getFunctionArity from "babel-helper-get-function-arity";
 import template from "babel-template";
 import * as t from "babel-types";
 
-let buildPropertyMethodAssignmentWrapper = template(`
+const buildPropertyMethodAssignmentWrapper = template(`
   (function (FUNCTION_KEY) {
     function FUNCTION_ID() {
       return FUNCTION_KEY.apply(this, arguments);
@@ -18,7 +18,7 @@ let buildPropertyMethodAssignmentWrapper = template(`
   })(FUNCTION)
 `);
 
-let buildGeneratorPropertyMethodAssignmentWrapper = template(`
+const buildGeneratorPropertyMethodAssignmentWrapper = template(`
   (function (FUNCTION_KEY) {
     function* FUNCTION_ID() {
       return yield* FUNCTION_KEY.apply(this, arguments);
@@ -32,14 +32,14 @@ let buildGeneratorPropertyMethodAssignmentWrapper = template(`
   })(FUNCTION)
 `);
 
-let visitor = {
+const visitor = {
   "ReferencedIdentifier|BindingIdentifier"(path, state) {
     // check if this node matches our function id
     if (path.node.name !== state.name) return;
 
     // check that we don't have a local variable declared as that removes the need
     // for the wrapper
-    let localDeclar = path.scope.getBindingIdentifier(state.name);
+    const localDeclar = path.scope.getBindingIdentifier(state.name);
     if (localDeclar !== state.outerDeclar) return;
 
     state.selfReference = true;
@@ -59,7 +59,7 @@ function wrap(state, method, id, scope) {
       // need to add a wrapper since we can't change the references
       let build = buildPropertyMethodAssignmentWrapper;
       if (method.generator) build = buildGeneratorPropertyMethodAssignmentWrapper;
-      let template = build({
+      const template = build({
         FUNCTION: method,
         FUNCTION_ID: id,
         FUNCTION_KEY: scope.generateUidIdentifier(id.name)
@@ -68,7 +68,7 @@ function wrap(state, method, id, scope) {
 
       // shim in dummy params to retain function arity, if you try to read the
       // source then you'll get the original since it's proxied so it's all good
-      let params = template.callee.body.body[0].params;
+      const params = template.callee.body.body[0].params;
       for (let i = 0, len = getFunctionArity(method); i < len; i++) {
         params.push(scope.generateUidIdentifier("x"));
       }
@@ -82,7 +82,7 @@ function wrap(state, method, id, scope) {
 }
 
 function visit(node, name, scope) {
-  let state = {
+  const state = {
     selfAssignment: false,
     selfReference:  false,
     outerDeclar:    scope.getBindingIdentifier(name),
@@ -93,7 +93,7 @@ function visit(node, name, scope) {
   // check to see if we have a local binding of the id we're setting inside of
   // the function, this is important as there are caveats associated
 
-  let binding = scope.getOwnBinding(name);
+  const binding = scope.getOwnBinding(name);
 
   if (binding) {
     if (binding.kind === "param") {
@@ -139,7 +139,7 @@ export default function ({ node, parent, scope, id }) {
     id = parent.id;
 
     if (t.isIdentifier(id)) {
-      let binding = scope.parent.getBinding(id.name);
+      const binding = scope.parent.getBinding(id.name);
       if (binding && binding.constant && scope.getBinding(id.name) === binding) {
         // always going to reference this method
         node.id = id;
@@ -171,6 +171,6 @@ export default function ({ node, parent, scope, id }) {
   // a local binding.
   id[t.NOT_LOCAL_BINDING] = true;
 
-  let state = visit(node, name, scope);
+  const state = visit(node, name, scope);
   return wrap(state, node, id, scope) || node;
 }
diff --git a/packages/babel-helper-get-function-arity/src/index.js b/packages/babel-helper-get-function-arity/src/index.js
index a3c60a5a614d..c54613f6fef3 100644
--- a/packages/babel-helper-get-function-arity/src/index.js
+++ b/packages/babel-helper-get-function-arity/src/index.js
@@ -1,9 +1,9 @@
 import * as t from "babel-types";
 
 export default function (node): number {
-  let params: Array<Object> = node.params;
+  const params: Array<Object> = node.params;
   for (let i = 0; i < params.length; i++) {
-    let param = params[i];
+    const param = params[i];
     if (t.isAssignmentPattern(param) || t.isRestElement(param)) {
       return i;
     }
diff --git a/packages/babel-helper-hoist-variables/src/index.js b/packages/babel-helper-hoist-variables/src/index.js
index de36a3f59197..dafcc98ca8e5 100644
--- a/packages/babel-helper-hoist-variables/src/index.js
+++ b/packages/babel-helper-hoist-variables/src/index.js
@@ -1,6 +1,6 @@
 import * as t from "babel-types";
 
-let visitor = {
+const visitor = {
   Scope(path, state) {
     if (state.kind === "let") path.skip();
   },
@@ -12,12 +12,12 @@ let visitor = {
   VariableDeclaration(path, state) {
     if (state.kind && path.node.kind !== state.kind) return;
 
-    let nodes = [];
+    const nodes = [];
 
-    let declarations: Array<Object> = path.get("declarations");
+    const declarations: Array<Object> = path.get("declarations");
     let firstId;
 
-    for (let declar of declarations) {
+    for (const declar of declarations) {
       firstId = declar.node.id;
 
       if (declar.node.init) {
@@ -26,7 +26,7 @@ let visitor = {
         ));
       }
 
-      for (let name in declar.getBindingIdentifiers()) {
+      for (const name in declar.getBindingIdentifiers()) {
         state.emit(t.identifier(name), name);
       }
     }
diff --git a/packages/babel-helper-plugin-test-runner/src/index.js b/packages/babel-helper-plugin-test-runner/src/index.js
index 749f964aa3da..37bcc4360c9f 100644
--- a/packages/babel-helper-plugin-test-runner/src/index.js
+++ b/packages/babel-helper-plugin-test-runner/src/index.js
@@ -2,6 +2,6 @@ import testRunner from "babel-helper-transform-fixture-test-runner";
 import path from "path";
 
 export default function (loc) {
-  let name = path.basename(path.dirname(loc));
+  const name = path.basename(path.dirname(loc));
   testRunner(loc + "/fixtures", name);
 }
diff --git a/packages/babel-helper-regex/src/index.js b/packages/babel-helper-regex/src/index.js
index b59a30f7d543..998bbe4716f7 100644
--- a/packages/babel-helper-regex/src/index.js
+++ b/packages/babel-helper-regex/src/index.js
@@ -6,7 +6,7 @@ export function is(node: Object, flag: string): boolean {
 }
 
 export function pullFlag(node: Object, flag: string) {
-  let flags = node.flags.split("");
+  const flags = node.flags.split("");
   if (node.flags.indexOf(flag) < 0) return;
   pull(flags, flag);
   node.flags = flags.join("");
diff --git a/packages/babel-helper-remap-async-to-generator/src/for-await.js b/packages/babel-helper-remap-async-to-generator/src/for-await.js
index 5e3fe1f0bc24..77b7738b7ca1 100644
--- a/packages/babel-helper-remap-async-to-generator/src/for-await.js
+++ b/packages/babel-helper-remap-async-to-generator/src/for-await.js
@@ -2,7 +2,7 @@ import * as t from "babel-types";
 import template from "babel-template";
 import traverse from "babel-traverse";
 
-let buildForAwait = template(`
+const buildForAwait = template(`
   function* wrapper() {
     var ITERATOR_COMPLETION = true;
     var ITERATOR_HAD_ERROR_KEY = false;
@@ -35,7 +35,7 @@ let buildForAwait = template(`
   }
 `);
 
-let forAwaitVisitor = {
+const forAwaitVisitor = {
   noScope: true,
 
   Identifier(path, replacements) {
@@ -45,7 +45,7 @@ let forAwaitVisitor = {
   },
 
   CallExpression(path, replacements) {
-    let callee = path.node.callee;
+    const callee = path.node.callee;
 
     // if no await wrapping is being applied, unwrap the call expression
     if (t.isIdentifier(callee) && callee.name === "AWAIT" && !replacements.AWAIT) {
@@ -55,11 +55,11 @@ let forAwaitVisitor = {
 };
 
 export default function (path, helpers) {
-  let { node, scope, parent } = path;
+  const { node, scope, parent } = path;
 
-  let stepKey = scope.generateUidIdentifier("step");
-  let stepValue = scope.generateUidIdentifier("value");
-  let left = node.left;
+  const stepKey = scope.generateUidIdentifier("step");
+  const stepValue = scope.generateUidIdentifier("value");
+  const left = node.left;
   let declar;
 
   if (t.isIdentifier(left) || t.isPattern(left) || t.isMemberExpression(left)) {
@@ -89,9 +89,9 @@ export default function (path, helpers) {
   // remove generator function wrapper
   template = template.body.body;
 
-  let isLabeledParent = t.isLabeledStatement(parent);
-  let tryBody = template[3].block.body;
-  let loop = tryBody[0];
+  const isLabeledParent = t.isLabeledStatement(parent);
+  const tryBody = template[3].block.body;
+  const loop = tryBody[0];
 
   if (isLabeledParent) {
     tryBody[0] = t.labeledStatement(parent.label, loop);
diff --git a/packages/babel-helper-remap-async-to-generator/src/index.js b/packages/babel-helper-remap-async-to-generator/src/index.js
index 840a195d8afd..deca1eb65087 100644
--- a/packages/babel-helper-remap-async-to-generator/src/index.js
+++ b/packages/babel-helper-remap-async-to-generator/src/index.js
@@ -6,7 +6,7 @@ import template from "babel-template";
 import * as t from "babel-types";
 import rewriteForAwait from "./for-await";
 
-let buildWrapper = template(`
+const buildWrapper = template(`
   (() => {
     var REF = FUNCTION;
     return function NAME(PARAMS) {
@@ -15,7 +15,7 @@ let buildWrapper = template(`
   })
 `);
 
-let namedBuildWrapper = template(`
+const namedBuildWrapper = template(`
   (() => {
     var REF = FUNCTION;
     function NAME(PARAMS) {
@@ -25,7 +25,7 @@ let namedBuildWrapper = template(`
   })
 `);
 
-let awaitVisitor = {
+const awaitVisitor = {
   Function(path) {
     if (path.isArrowFunctionExpression() && !path.node.async) {
       path.arrowFunctionToShadowed();
@@ -42,15 +42,15 @@ let awaitVisitor = {
   },
 
   ForAwaitStatement(path, { file, wrapAwait }) {
-    let { node } = path;
+    const { node } = path;
 
-    let build = rewriteForAwait(path, {
+    const build = rewriteForAwait(path, {
       getAsyncIterator: file.addHelper("asyncIterator"),
       wrapAwait
     });
 
-    let { declar, loop } = build;
-    let block = loop.body;
+    const { declar, loop } = build;
+    const block = loop.body;
 
     // ensure that it's a block so we can take all its statements
     path.ensureBlock();
@@ -77,12 +77,12 @@ let awaitVisitor = {
 };
 
 function classOrObjectMethod(path: NodePath, callId: Object) {
-  let node = path.node;
-  let body = node.body;
+  const node = path.node;
+  const body = node.body;
 
   node.async = false;
 
-  let container = t.functionExpression(null, [], t.blockStatement(body.body), true);
+  const container = t.functionExpression(null, [], t.blockStatement(body.body), true);
   container.shadow = true;
   body.body = [
     t.returnStatement(t.callExpression(
@@ -97,9 +97,9 @@ function classOrObjectMethod(path: NodePath, callId: Object) {
 }
 
 function plainFunction(path: NodePath, callId: Object) {
-  let node = path.node;
-  let isDeclaration = path.isFunctionDeclaration();
-  let asyncFnId = node.id;
+  const node = path.node;
+  const isDeclaration = path.isFunctionDeclaration();
+  const asyncFnId = node.id;
   let wrapper = buildWrapper;
 
   if (path.isArrowFunctionExpression()) {
@@ -117,8 +117,8 @@ function plainFunction(path: NodePath, callId: Object) {
     node.type = "FunctionExpression";
   }
 
-  let built = t.callExpression(callId, [node]);
-  let container = wrapper({
+  const built = t.callExpression(callId, [node]);
+  const container = wrapper({
     NAME: asyncFnId,
     REF: path.scope.generateUidIdentifier("ref"),
     FUNCTION: built,
@@ -137,7 +137,7 @@ function plainFunction(path: NodePath, callId: Object) {
   }).expression;
 
   if (isDeclaration) {
-    let declar = t.variableDeclaration("let", [
+    const declar = t.variableDeclaration("let", [
       t.variableDeclarator(
         t.identifier(asyncFnId.name),
         t.callExpression(container, [])
@@ -147,7 +147,7 @@ function plainFunction(path: NodePath, callId: Object) {
 
     path.replaceWith(declar);
   } else {
-    let retFunction = container.body.body[1].argument;
+    const retFunction = container.body.body[1].argument;
     if (!asyncFnId) {
       nameFunction({
         node: retFunction,
diff --git a/packages/babel-helper-replace-supers/src/index.js b/packages/babel-helper-replace-supers/src/index.js
index 8465df19361c..821475355213 100644
--- a/packages/babel-helper-replace-supers/src/index.js
+++ b/packages/babel-helper-replace-supers/src/index.js
@@ -6,7 +6,7 @@ import * as messages from "babel-messages";
 import * as t from "babel-types";
 
 // ✌️
-let HARDCORE_THIS_REF = Symbol();
+const HARDCORE_THIS_REF = Symbol();
 
 function isIllegalBareSuper(node, parent) {
   if (!t.isSuper(node)) return false;
@@ -46,7 +46,7 @@ function getPrototypeOfExpression(objectRef, isStatic) {
   );
 }
 
-let visitor = {
+const visitor = {
   Function(path) {
     if (!path.inShadow("this")) {
       path.skip();
@@ -69,9 +69,9 @@ let visitor = {
     let callback = state.specHandle;
     if (state.isLoose) callback = state.looseHandle;
 
-    let isBareSuper = path.isCallExpression() && path.get("callee").isSuper();
+    const isBareSuper = path.isCallExpression() && path.get("callee").isSuper();
 
-    let result = callback.call(state, path);
+    const result = callback.call(state, path);
 
     if (result) {
       state.hasSuper = true;
@@ -185,8 +185,8 @@ export default class ReplaceSupers {
   }
 
   getLooseSuperProperty(id: Object, parent: Object) {
-    let methodNode = this.methodNode;
-    let superRef   = this.superRef || t.identifier("Function");
+    const methodNode = this.methodNode;
+    const superRef   = this.superRef || t.identifier("Function");
 
     if (parent.property === id) {
       return;
@@ -201,11 +201,11 @@ export default class ReplaceSupers {
   }
 
   looseHandle(path: NodePath) {
-    let node = path.node;
+    const node = path.node;
     if (path.isSuper()) {
       return this.getLooseSuperProperty(node, path.parent);
     } else if (path.isCallExpression()) {
-      let callee = node.callee;
+      const callee = node.callee;
       if (!t.isMemberExpression(callee)) return;
       if (!t.isSuper(callee.object)) return;
 
@@ -239,15 +239,15 @@ export default class ReplaceSupers {
     let computed;
     let args;
 
-    let parent = path.parent;
-    let node = path.node;
+    const parent = path.parent;
+    const node = path.node;
 
     if (isIllegalBareSuper(node, parent)) {
       throw path.buildCodeFrameError(messages.get("classesIllegalBareSuper"));
     }
 
     if (t.isCallExpression(node)) {
-      let callee = node.callee;
+      const callee = node.callee;
       if (t.isSuper(callee)) {
         return;
       } else if (isMemberExpressionSuper(callee)) {
@@ -261,13 +261,13 @@ export default class ReplaceSupers {
       property = node.property;
       computed = node.computed;
     } else if (t.isUpdateExpression(node) && isMemberExpressionSuper(node.argument)) {
-      let binary = t.binaryExpression(node.operator[0], node.argument, t.numericLiteral(1));
+      const binary = t.binaryExpression(node.operator[0], node.argument, t.numericLiteral(1));
       if (node.prefix) {
         // ++super.foo; -> super.foo += 1;
         return this.specHandleAssignmentExpression(null, path, binary);
       } else {
         // super.foo++; -> let _ref = super.foo; super.foo = _ref + 1;
-        let ref = path.scope.generateUidIdentifier("ref");
+        const ref = path.scope.generateUidIdentifier("ref");
         return this.specHandleAssignmentExpression(ref, path, binary).concat(t.expressionStatement(ref));
       }
     } else if (t.isAssignmentExpression(node) && isMemberExpressionSuper(node.left)) {
@@ -276,7 +276,7 @@ export default class ReplaceSupers {
 
     if (!property) return;
 
-    let superProperty = this.getSuperProperty(property, computed);
+    const superProperty = this.getSuperProperty(property, computed);
 
     if (args) {
       return this.optimiseCall(superProperty, args);
@@ -286,7 +286,7 @@ export default class ReplaceSupers {
   }
 
   optimiseCall(callee, args) {
-    let thisNode = t.thisExpression();
+    const thisNode = t.thisExpression();
     thisNode[HARDCORE_THIS_REF] = true;
     return optimiseCall(callee, thisNode, args);
   }
diff --git a/packages/babel-helper-transform-fixture-test-runner/src/helpers.js b/packages/babel-helper-transform-fixture-test-runner/src/helpers.js
index bd4a61d3b986..f28904263838 100644
--- a/packages/babel-helper-transform-fixture-test-runner/src/helpers.js
+++ b/packages/babel-helper-transform-fixture-test-runner/src/helpers.js
@@ -16,4 +16,4 @@ export function multiline(arr) {
   return arr.join("\n");
 }
 
-export let assertArrayEquals = assert.deepEqual;
+export const assertArrayEquals = assert.deepEqual;
diff --git a/packages/babel-helper-transform-fixture-test-runner/src/index.js b/packages/babel-helper-transform-fixture-test-runner/src/index.js
index 9fced3069fae..11220e77ff19 100644
--- a/packages/babel-helper-transform-fixture-test-runner/src/index.js
+++ b/packages/babel-helper-transform-fixture-test-runner/src/index.js
@@ -11,7 +11,7 @@ import "babel-polyfill";
 import fs from "fs";
 import path from "path";
 
-let babelHelpers = eval(buildExternalHelpers(null, "var"));
+const babelHelpers = eval(buildExternalHelpers(null, "var"));
 
 function wrapPackagesArray(type, names, optionsDir) {
   return (names || []).map(function (val) {
@@ -36,14 +36,14 @@ function wrapPackagesArray(type, names, optionsDir) {
 }
 
 function run(task) {
-  let actual = task.actual;
-  let expect = task.expect;
-  let exec   = task.exec;
-  let opts   = task.options;
-  let optionsDir = task.optionsDir;
+  const actual = task.actual;
+  const expect = task.expect;
+  const exec   = task.exec;
+  const opts   = task.options;
+  const optionsDir = task.optionsDir;
 
   function getOpts(self) {
-    let newOpts = _.merge({
+    const newOpts = _.merge({
       filename: self.loc,
     }, opts);
 
@@ -64,8 +64,8 @@ function run(task) {
   let resultExec;
 
   if (execCode) {
-    let execOpts = getOpts(exec);
-    let execDirName = path.dirname(exec.loc);
+    const execOpts = getOpts(exec);
+    const execDirName = path.dirname(exec.loc);
     result = babel.transform(execCode, execOpts);
     execCode = result.code;
 
@@ -79,7 +79,7 @@ function run(task) {
   }
 
   let actualCode = actual.code;
-  let expectCode = expect.code;
+  const expectCode = expect.code;
   if (!execCode || actualCode) {
     result = babel.transform(actualCode, getOpts(actual));
     if (!expect.code && result.code && !opts.throws && fs.statSync(path.dirname(expect.loc)).isDirectory() && !process.env.CI) {
@@ -96,12 +96,12 @@ function run(task) {
   }
 
   if (task.sourceMappings) {
-    let consumer = new sourceMap.SourceMapConsumer(result.map);
+    const consumer = new sourceMap.SourceMapConsumer(result.map);
 
     _.each(task.sourceMappings, function (mapping) {
-      let actual = mapping.original;
+      const actual = mapping.original;
 
-      let expect = consumer.originalPositionFor(mapping.generated);
+      const expect = consumer.originalPositionFor(mapping.generated);
       chai.expect({ line: expect.line, column: expect.column }).to.deep.equal(actual);
     });
   }
@@ -112,7 +112,7 @@ function run(task) {
 }
 
 function runExec(opts, execCode, execDirname) {
-  let sandbox = {
+  const sandbox = {
     ...helpers,
     babelHelpers,
     assert: chai.assert,
@@ -124,7 +124,7 @@ function runExec(opts, execCode, execDirname) {
     }
   };
 
-  let fn = new Function(...Object.keys(sandbox), execCode);
+  const fn = new Function(...Object.keys(sandbox), execCode);
   return fn.apply(null, Object.values(sandbox));
 }
 
@@ -135,13 +135,13 @@ export default function (
   taskOpts = {},
   dynamicOpts?: Function,
 ) {
-  let suites = getFixtures(fixturesLoc);
+  const suites = getFixtures(fixturesLoc);
 
-  for (let testSuite of suites) {
+  for (const testSuite of suites) {
     if (_.includes(suiteOpts.ignoreSuites, testSuite.title)) continue;
 
     describe(name + "/" + testSuite.title, function () {
-      for (let task of testSuite.tests) {
+      for (const task of testSuite.tests) {
         if (_.includes(suiteOpts.ignoreTasks, task.title) ||
             _.includes(suiteOpts.ignoreTasks, testSuite.title + "/" + task.title)) continue;
 
@@ -163,7 +163,7 @@ export default function (
 
           if (dynamicOpts) dynamicOpts(task.options, task);
 
-          let throwMsg = task.options.throws;
+          const throwMsg = task.options.throws;
           if (throwMsg) {
             // internal api doesn't have this option but it's best not to pollute
             // the options object with useless options
@@ -174,7 +174,7 @@ export default function (
             });
           } else {
             if (task.exec.code) {
-              let result = run(task);
+              const result = run(task);
               if (result && typeof result.then === "function") {
                 return result;
               }
diff --git a/packages/babel-helpers/src/helpers.js b/packages/babel-helpers/src/helpers.js
index 87a58c3dc957..cdd63768da09 100644
--- a/packages/babel-helpers/src/helpers.js
+++ b/packages/babel-helpers/src/helpers.js
@@ -2,7 +2,7 @@
 
 import template from "babel-template";
 
-let helpers = {};
+const helpers = {};
 export default helpers;
 
 helpers.typeof = template(`
diff --git a/packages/babel-helpers/src/index.js b/packages/babel-helpers/src/index.js
index e742f224a368..207318cf275b 100644
--- a/packages/babel-helpers/src/index.js
+++ b/packages/babel-helpers/src/index.js
@@ -3,13 +3,13 @@
 import helpers from "./helpers";
 
 export function get(name) {
-  let fn = helpers[name];
+  const fn = helpers[name];
   if (!fn) throw new ReferenceError(`Unknown helper ${name}`);
 
   return fn().expression;
 }
 
-export let list = Object.keys(helpers)
+export const list = Object.keys(helpers)
   .map((name) => name[0] === "_" ? name.slice(1) : name)
   .filter((name) => name !== "__esModule");
 
diff --git a/packages/babel-messages/src/index.js b/packages/babel-messages/src/index.js
index 3fce2e2cf277..7064bd69b920 100644
--- a/packages/babel-messages/src/index.js
+++ b/packages/babel-messages/src/index.js
@@ -48,7 +48,7 @@ export const MESSAGES = {
  */
 
 export function get(key: string, ...args: Array<any>): string {
-  let msg = MESSAGES[key];
+  const msg = MESSAGES[key];
   if (!msg) throw new ReferenceError(`Unknown message ${JSON.stringify(key)}`);
 
   // stringify args
diff --git a/packages/babel-plugin-check-es2015-constants/src/index.js b/packages/babel-plugin-check-es2015-constants/src/index.js
index 4269b0385c7e..b090052bf59f 100644
--- a/packages/babel-plugin-check-es2015-constants/src/index.js
+++ b/packages/babel-plugin-check-es2015-constants/src/index.js
@@ -2,11 +2,11 @@ export default function ({ messages }) {
   return {
     visitor: {
       Scope({ scope }) {
-        for (let name in scope.bindings) {
-          let binding = scope.bindings[name];
+        for (const name in scope.bindings) {
+          const binding = scope.bindings[name];
           if (binding.kind !== "const" && binding.kind !== "module") continue;
 
-          for (let violation of (binding.constantViolations: Array)) {
+          for (const violation of (binding.constantViolations: Array)) {
             throw violation.buildCodeFrameError(messages.get("readOnly", name));
           }
         }
diff --git a/packages/babel-plugin-transform-async-generator-functions/src/index.js b/packages/babel-plugin-transform-async-generator-functions/src/index.js
index 409b14f36494..3da5e7d19b81 100644
--- a/packages/babel-plugin-transform-async-generator-functions/src/index.js
+++ b/packages/babel-plugin-transform-async-generator-functions/src/index.js
@@ -1,14 +1,14 @@
 import remapAsyncToGenerator from "babel-helper-remap-async-to-generator";
 
 export default function ({ types: t }) {
-  let yieldStarVisitor = {
+  const yieldStarVisitor = {
     Function(path) {
       path.skip();
     },
 
     YieldExpression({ node }, state) {
       if (!node.delegate) return;
-      let callee = state.addHelper("asyncGeneratorDelegate");
+      const callee = state.addHelper("asyncGeneratorDelegate");
       node.argument = t.callExpression(callee, [
         t.callExpression(state.addHelper("asyncIterator"), [node.argument]),
         t.memberExpression(state.addHelper("asyncGenerator"), t.identifier("await"))
diff --git a/packages/babel-plugin-transform-class-constructor-call/src/index.js b/packages/babel-plugin-transform-class-constructor-call/src/index.js
index 5d2e80f90f53..09de2b5baa80 100644
--- a/packages/babel-plugin-transform-class-constructor-call/src/index.js
+++ b/packages/babel-plugin-transform-class-constructor-call/src/index.js
@@ -1,6 +1,6 @@
 import template from "babel-template";
 
-let buildWrapper = template(`
+const buildWrapper = template(`
   let CLASS_REF = CLASS;
   var CALL_REF = CALL;
   var WRAPPER_REF = function (...args) {
@@ -15,12 +15,12 @@ let buildWrapper = template(`
 `);
 
 export default function ({ types: t }) {
-  let ALREADY_VISITED = Symbol();
+  const ALREADY_VISITED = Symbol();
 
   function findConstructorCall(path): ?Object {
-    let methods: Array<Object> = path.get("body.body");
+    const methods: Array<Object> = path.get("body.body");
 
-    for (let method of methods) {
+    for (const method of methods) {
       if (method.node.kind === "constructorCall") {
         return method;
       }
@@ -30,8 +30,8 @@ export default function ({ types: t }) {
   }
 
   function handleClassWithCall(constructorCall, classPath) {
-    let { node } = classPath;
-    let ref = node.id || classPath.scope.generateUidIdentifier("class");
+    const { node } = classPath;
+    const ref = node.id || classPath.scope.generateUidIdentifier("class");
 
     if (classPath.parentPath.isExportDefaultDeclaration()) {
       classPath = classPath.parentPath;
@@ -57,7 +57,7 @@ export default function ({ types: t }) {
         if (path.node[ALREADY_VISITED]) return;
         path.node[ALREADY_VISITED] = true;
 
-        let constructorCall = findConstructorCall(path);
+        const constructorCall = findConstructorCall(path);
 
         if (constructorCall) {
           handleClassWithCall(constructorCall, path);
diff --git a/packages/babel-plugin-transform-class-properties/src/index.js b/packages/babel-plugin-transform-class-properties/src/index.js
index 10451b377988..af383d44bd1f 100644
--- a/packages/babel-plugin-transform-class-properties/src/index.js
+++ b/packages/babel-plugin-transform-class-properties/src/index.js
@@ -3,7 +3,7 @@ import nameFunction from "babel-helper-function-name";
 import template from "babel-template";
 
 export default function ({ types: t }) {
-  let findBareSupers = {
+  const findBareSupers = {
     Super(path) {
       if (path.parentPath.isCallExpression({ callee: path.node })) {
         this.push(path.parentPath);
@@ -11,7 +11,7 @@ export default function ({ types: t }) {
     }
   };
 
-  let referenceVisitor = {
+  const referenceVisitor = {
     ReferencedIdentifier(path) {
       if (this.scope.hasOwnBinding(path.node.name)) {
         this.collision = true;
@@ -45,12 +45,12 @@ export default function ({ types: t }) {
     visitor: {
       Class(path, state) {
         const buildClassProperty = state.opts.spec ? buildClassPropertySpec : buildClassPropertyNonSpec;
-        let isDerived = !!path.node.superClass;
+        const isDerived = !!path.node.superClass;
         let constructor;
-        let props = [];
-        let body = path.get("body");
+        const props = [];
+        const body = path.get("body");
 
-        for (let path of body.get("body")) {
+        for (const path of body.get("body")) {
           if (path.isClassProperty()) {
             props.push(path);
           } else if (path.isClassMethod({ kind: "constructor" })) {
@@ -60,7 +60,7 @@ export default function ({ types: t }) {
 
         if (!props.length) return;
 
-        let nodes = [];
+        const nodes = [];
         let ref;
 
         if (path.isClassExpression() || !path.node.id) {
@@ -72,15 +72,15 @@ export default function ({ types: t }) {
 
         let instanceBody = [];
 
-        for (let prop of props) {
-          let propNode = prop.node;
+        for (const prop of props) {
+          const propNode = prop.node;
           if (propNode.decorators && propNode.decorators.length > 0) continue;
 
           // In non-spec mode, all properties without values are ignored.
           // In spec mode, *static* properties without values are still defined (see below).
           if (!state.opts.spec && !propNode.value) continue;
 
-          let isStatic = propNode.static;
+          const isStatic = propNode.static;
 
           if (isStatic) {
             nodes.push(buildClassProperty(ref, propNode));
@@ -92,7 +92,7 @@ export default function ({ types: t }) {
 
         if (instanceBody.length) {
           if (!constructor) {
-            let newConstructor = t.classMethod("constructor", t.identifier("constructor"), [], t.blockStatement([]));
+            const newConstructor = t.classMethod("constructor", t.identifier("constructor"), [], t.blockStatement([]));
             if (isDerived) {
               newConstructor.params = [t.restElement(t.identifier("args"))];
               newConstructor.body.body.push(
@@ -107,18 +107,18 @@ export default function ({ types: t }) {
             [constructor] = body.unshiftContainer("body", newConstructor);
           }
 
-          let collisionState = {
+          const collisionState = {
             collision: false,
             scope: constructor.scope
           };
 
-          for (let prop of props) {
+          for (const prop of props) {
             prop.traverse(referenceVisitor, collisionState);
             if (collisionState.collision) break;
           }
 
           if (collisionState.collision) {
-            let initialisePropsRef = path.scope.generateUidIdentifier("initialiseProps");
+            const initialisePropsRef = path.scope.generateUidIdentifier("initialiseProps");
 
             nodes.push(t.variableDeclaration("var", [
               t.variableDeclarator(
@@ -137,9 +137,9 @@ export default function ({ types: t }) {
           //
 
           if (isDerived) {
-            let bareSupers = [];
+            const bareSupers = [];
             constructor.traverse(findBareSupers, bareSupers);
-            for (let bareSuper of bareSupers) {
+            for (const bareSuper of bareSupers) {
               bareSuper.insertAfter(instanceBody);
             }
           } else {
@@ -147,7 +147,7 @@ export default function ({ types: t }) {
           }
         }
 
-        for (let prop of props) {
+        for (const prop of props) {
           prop.remove();
         }
 
@@ -169,11 +169,11 @@ export default function ({ types: t }) {
         path.insertAfter(nodes);
       },
       ArrowFunctionExpression(path) {
-        let classExp = path.get("body");
+        const classExp = path.get("body");
         if (!classExp.isClassExpression()) return;
 
-        let body = classExp.get("body");
-        let members = body.get("body");
+        const body = classExp.get("body");
+        const members = body.get("body");
         if (members.some((member) => member.isClassProperty())) {
           path.ensureBlock();
         }
diff --git a/packages/babel-plugin-transform-decorators/src/index.js b/packages/babel-plugin-transform-decorators/src/index.js
index 1b3fbed6f81c..e1156b690390 100644
--- a/packages/babel-plugin-transform-decorators/src/index.js
+++ b/packages/babel-plugin-transform-decorators/src/index.js
@@ -1,7 +1,7 @@
 import template from "babel-template";
 import explodeClass from "babel-helper-explode-class";
 
-let buildClassDecorator = template(`
+const buildClassDecorator = template(`
   CLASS_REF = DECORATOR(CLASS_REF) || CLASS_REF;
 `);
 
@@ -11,7 +11,7 @@ export default function ({ types: t }) {
   }
 
   function transformClass(path, ref, state) {
-    let nodes = [];
+    const nodes = [];
 
     state;
 
@@ -20,7 +20,7 @@ export default function ({ types: t }) {
       path.node.decorators = null;
       classDecorators = cleanDecorators(classDecorators);
 
-      for (let decorator of classDecorators) {
+      for (const decorator of classDecorators) {
         nodes.push(buildClassDecorator({
           CLASS_REF: ref,
           DECORATOR: decorator
@@ -28,21 +28,21 @@ export default function ({ types: t }) {
       }
     }
 
-    let map = Object.create(null);
+    const map = Object.create(null);
 
-    for (let method of path.get("body.body")) {
-      let decorators = method.node.decorators;
+    for (const method of path.get("body.body")) {
+      const decorators = method.node.decorators;
       if (!decorators) continue;
 
-      let alias = t.toKeyAlias(method.node);
+      const alias = t.toKeyAlias(method.node);
       map[alias] = map[alias] || [];
       map[alias].push(method.node);
 
       method.remove();
     }
 
-    for (let alias in map) {
-      let items = map[alias];
+    for (const alias in map) {
+      const items = map[alias];
 
       items;
     }
@@ -54,13 +54,13 @@ export default function ({ types: t }) {
     if (path.isClass()) {
       if (path.node.decorators) return true;
 
-      for (let method of (path.node.body.body: Array<Object>)) {
+      for (const method of (path.node.body.body: Array<Object>)) {
         if (method.decorators) {
           return true;
         }
       }
     } else if (path.isObjectExpression()) {
-      for (let prop of (path.node.properties: Array<Object>)) {
+      for (const prop of (path.node.properties: Array<Object>)) {
         if (prop.decorators) {
           return true;
         }
@@ -97,7 +97,7 @@ The repo url is: https://github.com/loganfsmyth/babel-plugin-transform-decorator
 
         explodeClass(path);
 
-        let ref = path.scope.generateDeclaredUidIdentifier("ref");
+        const ref = path.scope.generateDeclaredUidIdentifier("ref");
         let nodes = [];
 
         nodes.push(t.assignmentExpression("=", ref, path.node));
@@ -114,7 +114,7 @@ The repo url is: https://github.com/loganfsmyth/babel-plugin-transform-decorator
         doError(path);
         explodeClass(path);
 
-        let ref = path.node.id;
+        const ref = path.node.id;
         let nodes = [];
 
         nodes = nodes.concat(transformClass(path, ref, this).map((expr) => t.expressionStatement(expr)));
diff --git a/packages/babel-plugin-transform-do-expressions/src/index.js b/packages/babel-plugin-transform-do-expressions/src/index.js
index 8bfe9c0dc59b..1354c4ecfab9 100644
--- a/packages/babel-plugin-transform-do-expressions/src/index.js
+++ b/packages/babel-plugin-transform-do-expressions/src/index.js
@@ -4,7 +4,7 @@ export default function () {
 
     visitor: {
       DoExpression(path) {
-        let body = path.node.body.body;
+        const body = path.node.body.body;
         if (body.length) {
           path.replaceWithMultiple(body);
         } else {
diff --git a/packages/babel-plugin-transform-es2015-arrow-functions/src/index.js b/packages/babel-plugin-transform-es2015-arrow-functions/src/index.js
index 1d4c91fee75a..70b610355673 100644
--- a/packages/babel-plugin-transform-es2015-arrow-functions/src/index.js
+++ b/packages/babel-plugin-transform-es2015-arrow-functions/src/index.js
@@ -3,13 +3,13 @@ export default function ({ types: t }) {
     visitor: {
       ArrowFunctionExpression(path, state) {
         if (state.opts.spec) {
-          let { node } = path;
+          const { node } = path;
           if (node.shadow) return;
 
           node.shadow = { this: false };
           node.type = "FunctionExpression";
 
-          let boundThis = t.thisExpression();
+          const boundThis = t.thisExpression();
           boundThis._forceShadow = path;
 
           // make sure that arrow function won't be instantiated
diff --git a/packages/babel-plugin-transform-es2015-block-scoped-functions/src/index.js b/packages/babel-plugin-transform-es2015-block-scoped-functions/src/index.js
index a1aac15dc38c..54d27f224c3d 100644
--- a/packages/babel-plugin-transform-es2015-block-scoped-functions/src/index.js
+++ b/packages/babel-plugin-transform-es2015-block-scoped-functions/src/index.js
@@ -1,12 +1,12 @@
 export default function ({ types: t }) {
   function statementList(key, path) {
-    let paths: Array = path.get(key);
+    const paths: Array = path.get(key);
 
-    for (let path of paths) {
-      let func = path.node;
+    for (const path of paths) {
+      const func = path.node;
       if (!path.isFunctionDeclaration()) continue;
 
-      let declar = t.variableDeclaration("let", [
+      const declar = t.variableDeclaration("let", [
         t.variableDeclarator(func.id, t.toExpression(func))
       ]);
 
@@ -23,7 +23,7 @@ export default function ({ types: t }) {
   return {
     visitor: {
       BlockStatement(path) {
-        let { node, parent } = path;
+        const { node, parent } = path;
         if (t.isFunction(parent, { body: node })  || t.isExportDeclaration(parent)) {
           return;
         }
diff --git a/packages/babel-plugin-transform-es2015-block-scoping/src/index.js b/packages/babel-plugin-transform-es2015-block-scoping/src/index.js
index 95b63d36b363..19a36f558c16 100644
--- a/packages/babel-plugin-transform-es2015-block-scoping/src/index.js
+++ b/packages/babel-plugin-transform-es2015-block-scoping/src/index.js
@@ -14,17 +14,17 @@ export default function () {
   return {
     visitor: {
       VariableDeclaration(path, file) {
-        let { node, parent, scope } = path;
+        const { node, parent, scope } = path;
         if (!isBlockScoped(node)) return;
         convertBlockScopedToVar(path, null, parent, scope, true);
 
         if (node._tdzThis) {
-          let nodes = [node];
+          const nodes = [node];
 
           for (let i = 0; i < node.declarations.length; i++) {
-            let decl = node.declarations[i];
+            const decl = node.declarations[i];
             if (decl.init) {
-              let assign = t.assignmentExpression("=", decl.id, decl.init);
+              const assign = t.assignmentExpression("=", decl.id, decl.init);
               assign._ignoreBlockScopingTDZ = true;
               nodes.push(t.expressionStatement(assign));
             }
@@ -44,22 +44,22 @@ export default function () {
       },
 
       Loop(path, file) {
-        let { node, parent, scope } = path;
+        const { node, parent, scope } = path;
         t.ensureBlock(node);
-        let blockScoping = new BlockScoping(path, path.get("body"), parent, scope, file);
-        let replace = blockScoping.run();
+        const blockScoping = new BlockScoping(path, path.get("body"), parent, scope, file);
+        const replace = blockScoping.run();
         if (replace) path.replaceWith(replace);
       },
 
       CatchClause(path, file) {
-        let { parent, scope } = path;
-        let blockScoping = new BlockScoping(null, path.get("body"), parent, scope, file);
+        const { parent, scope } = path;
+        const blockScoping = new BlockScoping(null, path.get("body"), parent, scope, file);
         blockScoping.run();
       },
 
       "BlockStatement|SwitchStatement|Program"(path, file) {
         if (!ignoreBlock(path)) {
-          let blockScoping = new BlockScoping(null, path, path.parent, path.scope, file);
+          const blockScoping = new BlockScoping(null, path, path.parent, path.scope, file);
           blockScoping.run();
         }
       }
@@ -71,7 +71,7 @@ function ignoreBlock(path) {
   return t.isLoop(path.parent) || t.isCatchClause(path.parent);
 }
 
-let buildRetCheck = template(`
+const buildRetCheck = template(`
   if (typeof RETURN === "object") return RETURN.v;
 `);
 
@@ -89,7 +89,7 @@ function convertBlockScopedToVar(path, node, parent, scope, moveBindingsToParent
   // https://github.com/babel/babel/issues/255
   if (!t.isFor(parent)) {
     for (let i = 0; i < node.declarations.length; i++) {
-      let declar = node.declarations[i];
+      const declar = node.declarations[i];
       declar.init = declar.init || scope.buildUndefinedNode();
     }
   }
@@ -101,8 +101,8 @@ function convertBlockScopedToVar(path, node, parent, scope, moveBindingsToParent
   if (moveBindingsToParent) {
     const parentScope = scope.getFunctionParent();
     const ids = path.getBindingIdentifiers();
-    for (let name in ids) {
-      let binding = scope.getOwnBinding(name);
+    for (const name in ids) {
+      const binding = scope.getOwnBinding(name);
       if (binding) binding.kind = "var";
       scope.moveBindingTo(name, parentScope);
     }
@@ -113,36 +113,36 @@ function isVar(node) {
   return t.isVariableDeclaration(node, { kind: "var" }) && !isBlockScoped(node);
 }
 
-let letReferenceBlockVisitor = traverse.visitors.merge([{
+const letReferenceBlockVisitor = traverse.visitors.merge([{
   Function(path, state) {
     path.traverse(letReferenceFunctionVisitor, state);
     return path.skip();
   }
 }, tdzVisitor]);
 
-let letReferenceFunctionVisitor = traverse.visitors.merge([{
+const letReferenceFunctionVisitor = traverse.visitors.merge([{
   ReferencedIdentifier(path, state) {
-    let ref = state.letReferences[path.node.name];
+    const ref = state.letReferences[path.node.name];
 
     // not a part of our scope
     if (!ref) return;
 
     // this scope has a variable with the same name so it couldn't belong
     // to our let scope
-    let localBinding = path.scope.getBindingIdentifier(path.node.name);
+    const localBinding = path.scope.getBindingIdentifier(path.node.name);
     if (localBinding && localBinding !== ref) return;
 
     state.closurify = true;
   }
 }, tdzVisitor]);
 
-let hoistVarDeclarationsVisitor = {
+const hoistVarDeclarationsVisitor = {
   enter(path, self) {
-    let { node, parent } = path;
+    const { node, parent } = path;
 
     if (path.isForStatement()) {
       if (isVar(node.init, node)) {
-        let nodes = self.pushDeclar(node.init);
+        const nodes = self.pushDeclar(node.init);
         if (nodes.length === 1) {
           node.init = nodes[0];
         } else {
@@ -162,17 +162,17 @@ let hoistVarDeclarationsVisitor = {
   }
 };
 
-let loopLabelVisitor = {
+const loopLabelVisitor = {
   LabeledStatement({ node }, state) {
     state.innerLabels.push(node.label.name);
   }
 };
 
-let continuationVisitor = {
+const continuationVisitor = {
   enter(path, state) {
     if (path.isAssignmentExpression() || path.isUpdateExpression()) {
-      let bindings = path.getBindingIdentifiers();
-      for (let name in bindings) {
+      const bindings = path.getBindingIdentifiers();
+      for (const name in bindings) {
         if (state.outsideReferences[name] !== path.scope.getBindingIdentifier(name)) continue;
         state.reassignments[name] = true;
       }
@@ -188,9 +188,9 @@ function loopNodeTo(node) {
   }
 }
 
-let loopVisitor = {
+const loopVisitor = {
   Loop(path, state) {
-    let oldIgnoreLabeless = state.ignoreLabeless;
+    const oldIgnoreLabeless = state.ignoreLabeless;
     state.ignoreLabeless = true;
     path.traverse(loopVisitor, state);
     state.ignoreLabeless = oldIgnoreLabeless;
@@ -202,7 +202,7 @@ let loopVisitor = {
   },
 
   SwitchCase(path, state) {
-    let oldInSwitchCase = state.inSwitchCase;
+    const oldInSwitchCase = state.inSwitchCase;
     state.inSwitchCase = true;
     path.traverse(loopVisitor, state);
     state.inSwitchCase = oldInSwitchCase;
@@ -210,7 +210,7 @@ let loopVisitor = {
   },
 
   "BreakStatement|ContinueStatement|ReturnStatement"(path, state) {
-    let { node, parent, scope } = path;
+    const { node, parent, scope } = path;
     if (node[this.LOOP_IGNORE]) return;
 
     let replace;
@@ -284,11 +284,11 @@ class BlockScoping {
    */
 
   run() {
-    let block = this.block;
+    const block = this.block;
     if (block._letDone) return;
     block._letDone = true;
 
-    let needsClosure = this.getLetReferences();
+    const needsClosure = this.getLetReferences();
 
     // this is a block within a `Function/Program` so we can safely leave it be
     if (t.isFunction(this.parent) || t.isProgram(this.block)) {
@@ -313,12 +313,12 @@ class BlockScoping {
   }
 
   updateScopeInfo(wrappedInClosure) {
-    let scope = this.scope;
-    let parentScope = scope.getFunctionParent();
-    let letRefs = this.letReferences;
+    const scope = this.scope;
+    const parentScope = scope.getFunctionParent();
+    const letRefs = this.letReferences;
 
-    for (let key in letRefs) {
-      let ref = letRefs[key];
+    for (const key in letRefs) {
+      const ref = letRefs[key];
       const binding = scope.getBinding(ref.name);
       if (!binding) continue;
       if (binding.kind === "let" || binding.kind === "const") {
@@ -334,18 +334,18 @@ class BlockScoping {
   }
 
   remap() {
-    let letRefs   = this.letReferences;
-    let scope     = this.scope;
+    const letRefs   = this.letReferences;
+    const scope     = this.scope;
 
     // alright, so since we aren't wrapping this block in a closure
     // we have to check if any of our let variables collide with
     // those in upper scopes and then if they do, generate a uid
     // for them and replace all references with it
 
-    for (let key in letRefs) {
+    for (const key in letRefs) {
       // just an Identifier node we collected in `getLetReferences`
       // this is the defining identifier of a declaration
-      let ref = letRefs[key];
+      const ref = letRefs[key];
 
       // todo: could skip this if the colliding binding is in another function
       if (scope.parentHasBinding(key) || scope.hasGlobal(key)) {
@@ -362,14 +362,14 @@ class BlockScoping {
   }
 
   wrapClosure() {
-    let block = this.block;
+    const block = this.block;
 
-    let outsideRefs = this.outsideLetReferences;
+    const outsideRefs = this.outsideLetReferences;
 
     // remap loop heads with colliding variables
     if (this.loop) {
-      for (let name in outsideRefs) {
-        let id = outsideRefs[name];
+      for (const name in outsideRefs) {
+        const id = outsideRefs[name];
 
         if (this.scope.hasGlobal(id.name) || this.scope.parentHasBinding(id.name)) {
           delete outsideRefs[id.name];
@@ -391,13 +391,13 @@ class BlockScoping {
     this.hoistVarDeclarations();
 
     // turn outsideLetReferences into an array
-    let params = values(outsideRefs);
-    let args   = values(outsideRefs);
+    const params = values(outsideRefs);
+    const args   = values(outsideRefs);
 
     const isSwitch = this.blockPath.isSwitchStatement();
 
     // build the closure that we're going to wrap the block with, possible wrapping switch(){}
-    let fn = t.functionExpression(null, params, t.blockStatement(isSwitch ? [block] : block.body));
+    const fn = t.functionExpression(null, params, t.blockStatement(isSwitch ? [block] : block.body));
     fn.shadow = true;
 
     // continuation
@@ -414,17 +414,17 @@ class BlockScoping {
 
     // build a call and a unique id that we can assign the return value to
     let call = t.callExpression(ref, args);
-    let ret  = this.scope.generateUidIdentifier("ret");
+    const ret  = this.scope.generateUidIdentifier("ret");
 
     // handle generators
-    let hasYield = traverse.hasType(fn.body, this.scope, "YieldExpression", t.FUNCTION_TYPES);
+    const hasYield = traverse.hasType(fn.body, this.scope, "YieldExpression", t.FUNCTION_TYPES);
     if (hasYield) {
       fn.generator = true;
       call = t.yieldExpression(call, true);
     }
 
     // handlers async functions
-    let hasAsync = traverse.hasType(fn.body, this.scope, "AwaitExpression", t.FUNCTION_TYPES);
+    const hasAsync = traverse.hasType(fn.body, this.scope, "AwaitExpression", t.FUNCTION_TYPES);
     if (hasAsync) {
       fn.async = true;
       call = t.awaitExpression(call);
@@ -442,7 +442,7 @@ class BlockScoping {
    */
 
   buildClosure(ret: { type: "Identifier" }, call: { type: "CallExpression" }) {
-    let has = this.has;
+    const has = this.has;
     if (has.hasReturn || has.hasBreakContinue) {
       this.buildHas(ret, call);
     } else {
@@ -459,7 +459,7 @@ class BlockScoping {
    */
 
   addContinuations(fn) {
-    let state = {
+    const state = {
       reassignments: {},
       outsideReferences: this.outsideLetReferences
     };
@@ -467,10 +467,10 @@ class BlockScoping {
     this.scope.traverse(fn, continuationVisitor, state);
 
     for (let i = 0; i < fn.params.length; i++) {
-      let param = fn.params[i];
+      const param = fn.params[i];
       if (!state.reassignments[param.name]) continue;
 
-      let newParam = this.scope.generateUidIdentifier(param.name);
+      const newParam = this.scope.generateUidIdentifier(param.name);
       fn.params[i] = newParam;
 
       this.scope.rename(param.name, newParam.name, fn);
@@ -481,12 +481,12 @@ class BlockScoping {
   }
 
   getLetReferences() {
-    let block = this.block;
+    const block = this.block;
 
     let declarators = [];
 
     if (this.loop) {
-      let init = this.loop.left || this.loop.init;
+      const init = this.loop.left || this.loop.init;
       if (isBlockScoped(init)) {
         declarators.push(init);
         extend(this.outsideLetReferences, t.getBindingIdentifiers(init));
@@ -509,18 +509,18 @@ class BlockScoping {
     //
     if (block.body) {
       for (let i = 0; i < block.body.length; i++) {
-        let declarPath = this.blockPath.get("body")[i];
+        const declarPath = this.blockPath.get("body")[i];
         addDeclarationsFromChild(declarPath);
       }
     }
 
     if (block.cases) {
       for (let i = 0; i < block.cases.length; i++) {
-        let consequents = block.cases[i].consequent;
+        const consequents = block.cases[i].consequent;
 
         for (let j = 0; j < consequents.length; j++) {
-          let declarPath = this.blockPath.get("cases")[i];
-          let declar = consequents[j];
+          const declarPath = this.blockPath.get("cases")[i];
+          const declar = consequents[j];
           addDeclarationsFromChild(declarPath, declar);
         }
       }
@@ -528,12 +528,12 @@ class BlockScoping {
 
     //
     for (let i = 0; i < declarators.length; i++) {
-      let declar = declarators[i];
+      const declar = declarators[i];
       // Passing true as the third argument causes t.getBindingIdentifiers
       // to return only the *outer* binding identifiers of this
       // declaration, rather than (for example) mistakenly including the
       // parameters of a function declaration. Fixes #4880.
-      let keys = t.getBindingIdentifiers(declar, false, true);
+      const keys = t.getBindingIdentifiers(declar, false, true);
       extend(this.letReferences, keys);
       this.hasLetReferences = true;
     }
@@ -541,7 +541,7 @@ class BlockScoping {
     // no let references so we can just quit
     if (!this.hasLetReferences) return;
 
-    let state = {
+    const state = {
       letReferences: this.letReferences,
       closurify:     false,
       file:          this.file
@@ -562,7 +562,7 @@ class BlockScoping {
    */
 
   checkLoop(): Object {
-    let state = {
+    const state = {
       hasBreakContinue: false,
       ignoreLabeless:   false,
       inSwitchCase:     false,
@@ -594,21 +594,21 @@ class BlockScoping {
    */
 
   pushDeclar(node: { type: "VariableDeclaration" }): Array<Object> {
-    let declars = [];
-    let names = t.getBindingIdentifiers(node);
-    for (let name in names) {
+    const declars = [];
+    const names = t.getBindingIdentifiers(node);
+    for (const name in names) {
       declars.push(t.variableDeclarator(names[name]));
     }
 
     this.body.push(t.variableDeclaration(node.kind, declars));
 
-    let replace = [];
+    const replace = [];
 
     for (let i = 0; i < node.declarations.length; i++) {
-      let declar = node.declarations[i];
+      const declar = node.declarations[i];
       if (!declar.init) continue;
 
-      let expr = t.assignmentExpression("=", declar.id, declar.init);
+      const expr = t.assignmentExpression("=", declar.id, declar.init);
       replace.push(t.inherits(expr, declar));
     }
 
@@ -616,15 +616,15 @@ class BlockScoping {
   }
 
   buildHas(ret: { type: "Identifier" }, call: { type: "CallExpression" }) {
-    let body = this.body;
+    const body = this.body;
 
     body.push(t.variableDeclaration("var", [
       t.variableDeclarator(ret, call)
     ]));
 
     let retCheck;
-    let has = this.has;
-    let cases = [];
+    const has = this.has;
+    const cases = [];
 
     if (has.hasReturn) {
       // typeof ret === "object"
@@ -634,7 +634,7 @@ class BlockScoping {
     }
 
     if (has.hasBreakContinue) {
-      for (let key in has.map) {
+      for (const key in has.map) {
         cases.push(t.switchCase(t.stringLiteral(key), [has.map[key]]));
       }
 
@@ -643,7 +643,7 @@ class BlockScoping {
       }
 
       if (cases.length === 1) {
-        let single = cases[0];
+        const single = cases[0];
         body.push(t.ifStatement(
           t.binaryExpression("===", ret, single.test),
           single.consequent[0]
@@ -652,7 +652,7 @@ class BlockScoping {
         if (this.loop) {
           // https://github.com/babel/babel/issues/998
           for (let i = 0; i < cases.length; i++) {
-            let caseConsequent = cases[i].consequent[0];
+            const caseConsequent = cases[i].consequent[0];
             if (t.isBreakStatement(caseConsequent) && !caseConsequent.label) {
               caseConsequent.label = this.loopLabel = this.loopLabel || this.scope.generateUidIdentifier("loop");
             }
diff --git a/packages/babel-plugin-transform-es2015-block-scoping/src/tdz.js b/packages/babel-plugin-transform-es2015-block-scoping/src/tdz.js
index 10d68c174dca..17a3c2d21057 100644
--- a/packages/babel-plugin-transform-es2015-block-scoping/src/tdz.js
+++ b/packages/babel-plugin-transform-es2015-block-scoping/src/tdz.js
@@ -1,7 +1,7 @@
 import * as t from "babel-types";
 
 function getTDZStatus(refPath, bindingPath) {
-  let executionStatus = bindingPath._guessExecutionStatusRelativeTo(refPath);
+  const executionStatus = bindingPath._guessExecutionStatusRelativeTo(refPath);
 
   if (executionStatus === "before") {
     return "inside";
@@ -20,29 +20,29 @@ function buildTDZAssert(node, file) {
 }
 
 function isReference(node, scope, state) {
-  let declared = state.letReferences[node.name];
+  const declared = state.letReferences[node.name];
   if (!declared) return false;
 
   // declared node is different in this scope
   return scope.getBindingIdentifier(node.name) === declared;
 }
 
-export let visitor = {
+export const visitor = {
   ReferencedIdentifier(path, state) {
     if (!this.file.opts.tdz) return;
 
-    let { node, parent, scope } = path;
+    const { node, parent, scope } = path;
 
     if (path.parentPath.isFor({ left: node })) return;
     if (!isReference(node, scope, state)) return;
 
-    let bindingPath = scope.getBinding(node.name).path;
+    const bindingPath = scope.getBinding(node.name).path;
 
-    let status = getTDZStatus(path, bindingPath);
+    const status = getTDZStatus(path, bindingPath);
     if (status === "inside") return;
 
     if (status === "maybe") {
-      let assert = buildTDZAssert(node, state.file);
+      const assert = buildTDZAssert(node, state.file);
 
       // add tdzThis to parent variable declarator so it's exploded
       bindingPath.parent._tdzThis = true;
@@ -69,14 +69,14 @@ export let visitor = {
     exit(path, state) {
       if (!this.file.opts.tdz) return;
 
-      let { node } = path;
+      const { node } = path;
       if (node._ignoreBlockScopingTDZ) return;
 
-      let nodes = [];
-      let ids = path.getBindingIdentifiers();
+      const nodes = [];
+      const ids = path.getBindingIdentifiers();
 
-      for (let name in ids) {
-        let id = ids[name];
+      for (const name in ids) {
+        const id = ids[name];
 
         if (isReference(id, path.scope, state)) {
           nodes.push(buildTDZAssert(id, state.file));
diff --git a/packages/babel-plugin-transform-es2015-classes/src/index.js b/packages/babel-plugin-transform-es2015-classes/src/index.js
index 9b4d371e5c91..2dc8e3337d0b 100644
--- a/packages/babel-plugin-transform-es2015-classes/src/index.js
+++ b/packages/babel-plugin-transform-es2015-classes/src/index.js
@@ -4,15 +4,15 @@ import nameFunction from "babel-helper-function-name";
 
 export default function ({ types: t }) {
   // todo: investigate traversal requeueing
-  let VISITED = Symbol();
+  const VISITED = Symbol();
 
   return {
     visitor: {
       ExportDefaultDeclaration(path) {
         if (!path.get("declaration").isClassDeclaration()) return;
 
-        let { node } = path;
-        let ref = node.declaration.id || path.scope.generateUidIdentifier("class");
+        const { node } = path;
+        const ref = node.declaration.id || path.scope.generateUidIdentifier("class");
         node.declaration.id = ref;
 
         // Split the class declaration and the export into two separate statements.
@@ -21,9 +21,9 @@ export default function ({ types: t }) {
       },
 
       ClassDeclaration(path) {
-        let { node } = path;
+        const { node } = path;
 
-        let ref = node.id || path.scope.generateUidIdentifier("class");
+        const ref = node.id || path.scope.generateUidIdentifier("class");
 
         path.replaceWith(t.variableDeclaration("let", [
           t.variableDeclarator(ref, t.toExpression(node))
@@ -31,10 +31,10 @@ export default function ({ types: t }) {
       },
 
       ClassExpression(path, state) {
-        let { node } = path;
+        const { node } = path;
         if (node[VISITED]) return;
 
-        let inferred = nameFunction(path);
+        const inferred = nameFunction(path);
         if (inferred && inferred !== node) return path.replaceWith(inferred);
 
         node[VISITED] = true;
diff --git a/packages/babel-plugin-transform-es2015-classes/src/lib/memoise-decorators.js b/packages/babel-plugin-transform-es2015-classes/src/lib/memoise-decorators.js
index 294852818264..6583d081dc64 100644
--- a/packages/babel-plugin-transform-es2015-classes/src/lib/memoise-decorators.js
+++ b/packages/babel-plugin-transform-es2015-classes/src/lib/memoise-decorators.js
@@ -2,14 +2,14 @@ import type { Scope } from "babel-traverse";
 import * as t from "babel-types";
 
 export default function (decorators: Array<Object>, scope: Scope): Array<Object> {
-  for (let decorator of decorators) {
-    let expression = decorator.expression;
+  for (const decorator of decorators) {
+    const expression = decorator.expression;
     if (!t.isMemberExpression(expression)) continue;
 
-    let temp = scope.maybeGenerateMemoised(expression.object);
+    const temp = scope.maybeGenerateMemoised(expression.object);
     let ref;
 
-    let nodes = [];
+    const nodes = [];
 
     if (temp) {
       ref = temp;
diff --git a/packages/babel-plugin-transform-es2015-classes/src/loose.js b/packages/babel-plugin-transform-es2015-classes/src/loose.js
index 5c9122bacc0c..b94cd6be0516 100644
--- a/packages/babel-plugin-transform-es2015-classes/src/loose.js
+++ b/packages/babel-plugin-transform-es2015-classes/src/loose.js
@@ -14,11 +14,11 @@ export default class LooseClassTransformer extends VanillaTransformer {
 
       let classRef = this.classRef;
       if (!node.static) classRef = t.memberExpression(classRef, t.identifier("prototype"));
-      let methodName = t.memberExpression(classRef, node.key, node.computed || t.isLiteral(node.key));
+      const methodName = t.memberExpression(classRef, node.key, node.computed || t.isLiteral(node.key));
 
       let func = t.functionExpression(null, node.params, node.body, node.generator, node.async);
       func.returnType = node.returnType;
-      let key = t.toComputedKey(node, node.key);
+      const key = t.toComputedKey(node, node.key);
       if (t.isStringLiteral(key)) {
         func = nameFunction({
           node: func,
@@ -27,7 +27,7 @@ export default class LooseClassTransformer extends VanillaTransformer {
         });
       }
 
-      let expr = t.expressionStatement(t.assignmentExpression("=", methodName, func));
+      const expr = t.expressionStatement(t.assignmentExpression("=", methodName, func));
       t.inheritsComments(expr, node);
       this.body.push(expr);
       return true;
diff --git a/packages/babel-plugin-transform-es2015-classes/src/vanilla.js b/packages/babel-plugin-transform-es2015-classes/src/vanilla.js
index 34109568af80..b38a00fa0265 100644
--- a/packages/babel-plugin-transform-es2015-classes/src/vanilla.js
+++ b/packages/babel-plugin-transform-es2015-classes/src/vanilla.js
@@ -8,13 +8,13 @@ import * as defineMap from "babel-helper-define-map";
 import template from "babel-template";
 import * as t from "babel-types";
 
-let buildDerivedConstructor = template(`
+const buildDerivedConstructor = template(`
   (function () {
     super(...arguments);
   })
 `);
 
-let noMethodVisitor = {
+const noMethodVisitor = {
   "FunctionExpression|FunctionDeclaration"(path) {
     if (!path.is("shadow")) {
       path.skip();
@@ -26,7 +26,7 @@ let noMethodVisitor = {
   }
 };
 
-let verifyConstructorVisitor = visitors.merge([noMethodVisitor, {
+const verifyConstructorVisitor = visitors.merge([noMethodVisitor, {
   Super(path) {
     if (this.isDerived && !this.hasBareSuper && !path.parentPath.isCallExpression({ callee: path.node })) {
       throw path.buildCodeFrameError("'super.*' is not allowed before super()");
@@ -54,7 +54,7 @@ let verifyConstructorVisitor = visitors.merge([noMethodVisitor, {
   }
 }]);
 
-let findThisesVisitor = visitors.merge([noMethodVisitor, {
+const findThisesVisitor = visitors.merge([noMethodVisitor, {
   ThisExpression(path) {
     this.superThises.push(path);
   }
@@ -96,18 +96,18 @@ export default class ClassTransformer {
 
   run() {
     let superName = this.superName;
-    let file      = this.file;
+    const file      = this.file;
     let body      = this.body;
 
     //
 
-    let constructorBody = this.constructorBody = t.blockStatement([]);
+    const constructorBody = this.constructorBody = t.blockStatement([]);
     this.constructor    = this.buildConstructor();
 
     //
 
-    let closureParams = [];
-    let closureArgs = [];
+    const closureParams = [];
+    const closureArgs = [];
 
     //
     if (this.isDerived) {
@@ -138,13 +138,13 @@ export default class ClassTransformer {
     //
     body.push(t.returnStatement(this.classRef));
 
-    let container = t.functionExpression(null, closureParams, t.blockStatement(body));
+    const container = t.functionExpression(null, closureParams, t.blockStatement(body));
     container.shadow = true;
     return t.callExpression(container, closureArgs);
   }
 
   buildConstructor() {
-    let func = t.functionDeclaration(this.classRef, [], this.constructorBody);
+    const func = t.functionDeclaration(this.classRef, [], this.constructorBody);
     t.inherits(func, this.node);
     return func;
   }
@@ -159,7 +159,7 @@ export default class ClassTransformer {
       mutatorMap = this.instanceMutatorMap;
     }
 
-    let map = defineMap.push(mutatorMap, node, kind, this.file, scope);
+    const map = defineMap.push(mutatorMap, node, kind, this.file, scope);
 
     if (enumerable) {
       map.enumerable = t.booleanLiteral(true);
@@ -175,8 +175,8 @@ export default class ClassTransformer {
 
   constructorMeMaybe() {
     let hasConstructor = false;
-    let paths = this.path.get("body.body");
-    for (let path of (paths: Array)) {
+    const paths = this.path.get("body.body");
+    for (const path of (paths: Array)) {
       hasConstructor = path.equals("kind", "constructor");
       if (hasConstructor) break;
     }
@@ -185,7 +185,7 @@ export default class ClassTransformer {
     let params, body;
 
     if (this.isDerived) {
-      let constructor = buildDerivedConstructor().expression;
+      const constructor = buildDerivedConstructor().expression;
       params = constructor.params;
       body = constructor.body;
     } else {
@@ -207,7 +207,7 @@ export default class ClassTransformer {
     this.verifyConstructor();
 
     if (this.userConstructor) {
-      let constructorBody = this.constructorBody;
+      const constructorBody = this.constructorBody;
       constructorBody.body = constructorBody.body.concat(this.userConstructor.body.body);
       t.inherits(this.constructor, this.userConstructor);
       t.inherits(constructorBody, this.userConstructor.body);
@@ -217,10 +217,10 @@ export default class ClassTransformer {
   }
 
   pushBody() {
-    let classBodyPaths: Array<Object> = this.path.get("body.body");
+    const classBodyPaths: Array<Object> = this.path.get("body.body");
 
-    for (let path of classBodyPaths) {
-      let node = path.node;
+    for (const path of classBodyPaths) {
+      const node = path.node;
 
       if (path.isClassProperty()) {
         throw path.buildCodeFrameError("Missing class properties transform.");
@@ -231,7 +231,7 @@ export default class ClassTransformer {
       }
 
       if (t.isClassMethod(node)) {
-        let isConstructor = node.kind === "constructor";
+        const isConstructor = node.kind === "constructor";
 
         if (isConstructor) {
           path.traverse(verifyConstructorVisitor, this);
@@ -241,7 +241,7 @@ export default class ClassTransformer {
           }
         }
 
-        let replaceSupers = new ReplaceSupers({
+        const replaceSupers = new ReplaceSupers({
           forceSuperMemoisation: isConstructor,
           methodPath:            path,
           methodNode:            node,
@@ -275,7 +275,7 @@ export default class ClassTransformer {
   pushDescriptors() {
     this.pushInherits();
 
-    let body = this.body;
+    const body = this.body;
 
     let instanceProps;
     let staticProps;
@@ -292,7 +292,7 @@ export default class ClassTransformer {
       if (instanceProps) instanceProps = defineMap.toComputedObjectFromClass(instanceProps);
       if (staticProps) staticProps = defineMap.toComputedObjectFromClass(staticProps);
 
-      let nullNode = t.nullLiteral();
+      const nullNode = t.nullLiteral();
 
       // (Constructor, instanceDescriptors, staticDescriptors, instanceInitializers, staticInitializers)
       let args = [this.classRef, nullNode, nullNode, nullNode, nullNode];
@@ -363,7 +363,7 @@ export default class ClassTransformer {
       [t.thisExpression(), bareSuperNode]
     );
 
-    let bareSuperAfter = this.bareSuperAfter.map((fn) => fn(thisRef));
+    const bareSuperAfter = this.bareSuperAfter.map((fn) => fn(thisRef));
 
     if (bareSuper.parentPath.isExpressionStatement() && bareSuper.parentPath.container === body.node.body && body.node.body.length - 1 === bareSuper.parentPath.key) {
       // this super call is the last statement in the body so we can just straight up
@@ -394,17 +394,17 @@ export default class ClassTransformer {
   verifyConstructor() {
     if (!this.isDerived) return;
 
-    let path = this.userConstructorPath;
-    let body = path.get("body");
+    const path = this.userConstructorPath;
+    const body = path.get("body");
 
     path.traverse(findThisesVisitor, this);
 
     let guaranteedSuperBeforeFinish = !!this.bareSupers.length;
 
-    let superRef = this.superName || t.identifier("Function");
-    let thisRef = path.scope.generateUidIdentifier("this");
+    const superRef = this.superName || t.identifier("Function");
+    const thisRef = path.scope.generateUidIdentifier("this");
 
-    for (let bareSuper of this.bareSupers) {
+    for (const bareSuper of this.bareSupers) {
       this.wrapSuperCall(bareSuper, superRef, thisRef, body);
 
       if (guaranteedSuperBeforeFinish) {
@@ -422,25 +422,25 @@ export default class ClassTransformer {
       }
     }
 
-    for (let thisPath of this.superThises) {
+    for (const thisPath of this.superThises) {
       thisPath.replaceWith(thisRef);
     }
 
-    let wrapReturn = (returnArg) => t.callExpression(
+    const wrapReturn = (returnArg) => t.callExpression(
       this.file.addHelper("possibleConstructorReturn"),
       [thisRef].concat(returnArg || [])
     );
 
     // if we have a return as the last node in the body then we've already caught that
     // return
-    let bodyPaths = body.get("body");
+    const bodyPaths = body.get("body");
     if (bodyPaths.length && !bodyPaths.pop().isReturnStatement()) {
       body.pushContainer("body", t.returnStatement(guaranteedSuperBeforeFinish ? thisRef : wrapReturn()));
     }
 
-    for (let returnPath of this.superReturns) {
+    for (const returnPath of this.superReturns) {
       if (returnPath.node.argument) {
-        let ref = returnPath.scope.generateDeclaredUidIdentifier("ret");
+        const ref = returnPath.scope.generateDeclaredUidIdentifier("ret");
         returnPath.get("argument").replaceWithMultiple([
           t.assignmentExpression("=", ref, returnPath.node.argument),
           wrapReturn(ref)
@@ -456,7 +456,7 @@ export default class ClassTransformer {
    */
 
   pushMethod(node: { type: "ClassMethod" }, path?: NodePath) {
-    let scope = path ? path.scope : this.scope;
+    const scope = path ? path.scope : this.scope;
 
     if (node.kind === "method") {
       if (this._processMethod(node, scope)) return;
@@ -482,7 +482,7 @@ export default class ClassTransformer {
       path.scope.rename(this.classRef.name);
     }
 
-    let construct = this.constructor;
+    const construct = this.constructor;
 
     this.userConstructorPath = path;
     this.userConstructor     = method;
diff --git a/packages/babel-plugin-transform-es2015-computed-properties/src/index.js b/packages/babel-plugin-transform-es2015-computed-properties/src/index.js
index 6bf843aaab7e..ac0ef38fc8b0 100644
--- a/packages/babel-plugin-transform-es2015-computed-properties/src/index.js
+++ b/packages/babel-plugin-transform-es2015-computed-properties/src/index.js
@@ -1,5 +1,5 @@
 export default function ({ types: t, template }) {
-  let buildMutatorMapAssign = template(`
+  const buildMutatorMapAssign = template(`
     MUTATOR_MAP_REF[KEY] = MUTATOR_MAP_REF[KEY] || {};
     MUTATOR_MAP_REF[KEY].KIND = VALUE;
   `);
@@ -29,7 +29,7 @@ export default function ({ types: t, template }) {
   function pushMutatorDefine({ objId, body, getMutatorId, scope }, prop) {
     let key = !prop.computed && t.isIdentifier(prop.key) ? t.stringLiteral(prop.key.name) : prop.key;
 
-    let maybeMemoise = scope.maybeGenerateMemoised(key);
+    const maybeMemoise = scope.maybeGenerateMemoised(key);
     if (maybeMemoise) {
       body.push(t.expressionStatement(t.assignmentExpression("=", maybeMemoise, key)));
       key = maybeMemoise;
@@ -44,7 +44,7 @@ export default function ({ types: t, template }) {
   }
 
   function loose(info) {
-    for (let prop of info.computedProps) {
+    for (const prop of info.computedProps) {
       if (prop.kind === "get" || prop.kind === "set") {
         pushMutatorDefine(info, prop);
       } else {
@@ -54,10 +54,10 @@ export default function ({ types: t, template }) {
   }
 
   function spec(info) {
-    let { objId, body, computedProps, state } = info;
+    const { objId, body, computedProps, state } = info;
 
-    for (let prop of computedProps) {
-      let key = t.toComputedKey(prop);
+    for (const prop of computedProps) {
+      const key = t.toComputedKey(prop);
 
       if (prop.kind === "get" || prop.kind === "set") {
         pushMutatorDefine(info, prop);
@@ -87,9 +87,9 @@ export default function ({ types: t, template }) {
     visitor: {
       ObjectExpression: {
         exit(path, state) {
-          let { node, parent, scope } = path;
+          const { node, parent, scope } = path;
           let hasComputed = false;
-          for (let prop of (node.properties: Array<Object>)) {
+          for (const prop of (node.properties: Array<Object>)) {
             hasComputed = prop.computed === true;
             if (hasComputed) break;
           }
@@ -98,11 +98,11 @@ export default function ({ types: t, template }) {
           // put all getters/setters into the first object expression as well as all initialisers up
           // to the first computed property
 
-          let initProps = [];
-          let computedProps = [];
+          const initProps = [];
+          const computedProps = [];
           let foundComputed = false;
 
-          for (let prop of node.properties) {
+          for (const prop of node.properties) {
             if (prop.computed) {
               foundComputed = true;
             }
@@ -114,9 +114,9 @@ export default function ({ types: t, template }) {
             }
           }
 
-          let objId = scope.generateUidIdentifierBasedOnNode(parent);
-          let initPropExpression = t.objectExpression(initProps);
-          let body = [];
+          const objId = scope.generateUidIdentifierBasedOnNode(parent);
+          const initPropExpression = t.objectExpression(initProps);
+          const body = [];
 
           body.push(t.variableDeclaration("var", [
             t.variableDeclarator(objId, initPropExpression)
@@ -127,7 +127,7 @@ export default function ({ types: t, template }) {
 
           let mutatorRef;
 
-          let getMutatorId = function () {
+          const getMutatorId = function () {
             if (!mutatorRef) {
               mutatorRef = scope.generateUidIdentifier("mutatorMap");
 
@@ -139,7 +139,7 @@ export default function ({ types: t, template }) {
             return mutatorRef;
           };
 
-          let single = callback({
+          const single = callback({
             scope,
             objId,
             body,
diff --git a/packages/babel-plugin-transform-es2015-destructuring/src/index.js b/packages/babel-plugin-transform-es2015-destructuring/src/index.js
index 3fe8c9fd3bf7..6184cf2547e6 100644
--- a/packages/babel-plugin-transform-es2015-destructuring/src/index.js
+++ b/packages/babel-plugin-transform-es2015-destructuring/src/index.js
@@ -7,7 +7,7 @@ export default function ({ types: t }) {
    */
 
   function variableDeclarationHasPattern(node) {
-    for (let declar of (node.declarations: Array)) {
+    for (const declar of (node.declarations: Array)) {
       if (t.isPattern(declar.id)) {
         return true;
       }
@@ -20,7 +20,7 @@ export default function ({ types: t }) {
    */
 
   function hasRest(pattern) {
-    for (let elem of (pattern.elements: Array)) {
+    for (const elem of (pattern.elements: Array)) {
       if (t.isRestElement(elem)) {
         return true;
       }
@@ -28,7 +28,7 @@ export default function ({ types: t }) {
     return false;
   }
 
-  let arrayUnpackVisitor = {
+  const arrayUnpackVisitor = {
     ReferencedIdentifier(path, state) {
       if (state.bindings[path.node.name]) {
         state.deopt = true;
@@ -68,7 +68,7 @@ export default function ({ types: t }) {
     }
 
     buildVariableDeclaration(id, init) {
-      let declar = t.variableDeclaration("var", [
+      const declar = t.variableDeclaration("var", [
         t.variableDeclarator(id, init)
       ]);
       declar._blockHoist = this.blockHoist;
@@ -99,9 +99,9 @@ export default function ({ types: t }) {
       // we need to assign the current value of the assignment to avoid evaluating
       // it more than once
 
-      let tempValueRef = this.scope.generateUidIdentifierBasedOnNode(valueRef);
+      const tempValueRef = this.scope.generateUidIdentifierBasedOnNode(valueRef);
 
-      let declar = t.variableDeclaration("var", [
+      const declar = t.variableDeclaration("var", [
         t.variableDeclarator(tempValueRef, valueRef)
       ]);
       declar._blockHoist = this.blockHoist;
@@ -109,15 +109,15 @@ export default function ({ types: t }) {
 
       //
 
-      let tempConditional = t.conditionalExpression(
+      const tempConditional = t.conditionalExpression(
         t.binaryExpression("===", tempValueRef, t.identifier("undefined")),
         pattern.right,
         tempValueRef
       );
 
-      let left = pattern.left;
+      const left = pattern.left;
       if (t.isPattern(left)) {
-        let tempValueDefault = t.expressionStatement(
+        const tempValueDefault = t.expressionStatement(
           t.assignmentExpression("=", tempValueRef, tempConditional)
         );
         tempValueDefault._blockHoist = this.blockHoist;
@@ -135,7 +135,7 @@ export default function ({ types: t }) {
       let keys = [];
 
       for (let i = 0; i < pattern.properties.length; i++) {
-        let prop = pattern.properties[i];
+        const prop = pattern.properties[i];
 
         // we've exceeded the index of the spread property to all properties to the
         // right need to be ignored
@@ -153,15 +153,15 @@ export default function ({ types: t }) {
 
       //
 
-      let value = t.callExpression(this.file.addHelper("objectWithoutProperties"), [objRef, keys]);
+      const value = t.callExpression(this.file.addHelper("objectWithoutProperties"), [objRef, keys]);
       this.nodes.push(this.buildVariableAssignment(spreadProp.argument, value));
     }
 
     pushObjectProperty(prop, propRef) {
       if (t.isLiteral(prop.key)) prop.computed = true;
 
-      let pattern = prop.value;
-      let objRef  = t.memberExpression(propRef, prop.key, prop.computed);
+      const pattern = prop.value;
+      const objRef  = t.memberExpression(propRef, prop.key, prop.computed);
 
       if (t.isPattern(pattern)) {
         this.push(pattern, objRef);
@@ -184,7 +184,7 @@ export default function ({ types: t }) {
       // only evaluated once
 
       if (pattern.properties.length > 1 && !this.scope.isStatic(objRef)) {
-        let temp = this.scope.generateUidIdentifierBasedOnNode(objRef);
+        const temp = this.scope.generateUidIdentifierBasedOnNode(objRef);
         this.nodes.push(this.buildVariableDeclaration(temp, objRef));
         objRef = temp;
       }
@@ -192,7 +192,7 @@ export default function ({ types: t }) {
       //
 
       for (let i = 0; i < pattern.properties.length; i++) {
-        let prop = pattern.properties[i];
+        const prop = pattern.properties[i];
         if (t.isRestProperty(prop)) {
           this.pushObjectRest(pattern, objRef, prop, i);
         } else {
@@ -210,7 +210,7 @@ export default function ({ types: t }) {
       if (pattern.elements.length > arr.elements.length) return;
       if (pattern.elements.length < arr.elements.length && !hasRest(pattern)) return false;
 
-      for (let elem of (pattern.elements: Array)) {
+      for (const elem of (pattern.elements: Array)) {
         // deopt on holes
         if (!elem) return false;
 
@@ -218,7 +218,7 @@ export default function ({ types: t }) {
         if (t.isMemberExpression(elem)) return false;
       }
 
-      for (let elem of (arr.elements: Array)) {
+      for (const elem of (arr.elements: Array)) {
         // deopt on spread elements
         if (t.isSpreadElement(elem)) return false;
 
@@ -230,15 +230,15 @@ export default function ({ types: t }) {
       }
 
       // deopt on reference to left side identifiers
-      let bindings = t.getBindingIdentifiers(pattern);
-      let state = { deopt: false, bindings };
+      const bindings = t.getBindingIdentifiers(pattern);
+      const state = { deopt: false, bindings };
       this.scope.traverse(arr, arrayUnpackVisitor, state);
       return !state.deopt;
     }
 
     pushUnpackedArrayPattern(pattern, arr) {
       for (let i = 0; i < pattern.elements.length; i++) {
-        let elem = pattern.elements[i];
+        const elem = pattern.elements[i];
         if (t.isRestElement(elem)) {
           this.push(elem.argument, t.arrayExpression(arr.elements.slice(i)));
         } else {
@@ -264,13 +264,13 @@ export default function ({ types: t }) {
       // if we have a rest then we need all the elements so don't tell
       // `scope.toArray` to only get a certain amount
 
-      let count = !hasRest(pattern) && pattern.elements.length;
+      const count = !hasRest(pattern) && pattern.elements.length;
 
       // so we need to ensure that the `arrayRef` is an array, `scope.toArray` will
       // return a locally bound identifier if it's been inferred to be an array,
       // otherwise it'll be a call to a helper that will ensure it's one
 
-      let toArray = this.toArray(arrayRef, count);
+      const toArray = this.toArray(arrayRef, count);
 
       if (t.isIdentifier(toArray)) {
         // we've been given an identifier so it must have been inferred to be an
@@ -315,7 +315,7 @@ export default function ({ types: t }) {
       // need to save it to a variable
 
       if (!t.isArrayExpression(ref) && !t.isMemberExpression(ref)) {
-        let memo = this.scope.maybeGenerateMemoised(ref, true);
+        const memo = this.scope.maybeGenerateMemoised(ref, true);
         if (memo) {
           this.nodes.push(this.buildVariableDeclaration(memo, ref));
           ref = memo;
@@ -334,14 +334,14 @@ export default function ({ types: t }) {
   return {
     visitor: {
       ExportNamedDeclaration(path) {
-        let declaration = path.get("declaration");
+        const declaration = path.get("declaration");
         if (!declaration.isVariableDeclaration()) return;
         if (!variableDeclarationHasPattern(declaration.node)) return;
 
-        let specifiers = [];
+        const specifiers = [];
 
-        for (let name in path.getOuterBindingIdentifiers(path)) {
-          let id = t.identifier(name);
+        for (const name in path.getOuterBindingIdentifiers(path)) {
+          const id = t.identifier(name);
           specifiers.push(t.exportSpecifier(id, id));
         }
 
@@ -353,13 +353,13 @@ export default function ({ types: t }) {
       },
 
       ForXStatement(path, file) {
-        let { node, scope } = path;
-        let left = node.left;
+        const { node, scope } = path;
+        const left = node.left;
 
         if (t.isPattern(left)) {
           // for ({ length: k } in { abc: 3 });
 
-          let temp = scope.generateUidIdentifier("ref");
+          const temp = scope.generateUidIdentifier("ref");
 
           node.left = t.variableDeclaration("var", [
             t.variableDeclarator(temp)
@@ -376,17 +376,17 @@ export default function ({ types: t }) {
 
         if (!t.isVariableDeclaration(left)) return;
 
-        let pattern = left.declarations[0].id;
+        const pattern = left.declarations[0].id;
         if (!t.isPattern(pattern)) return;
 
-        let key = scope.generateUidIdentifier("ref");
+        const key = scope.generateUidIdentifier("ref");
         node.left = t.variableDeclaration(left.kind, [
           t.variableDeclarator(key, null)
         ]);
 
-        let nodes = [];
+        const nodes = [];
 
-        let destructuring = new DestructuringTransformer({
+        const destructuring = new DestructuringTransformer({
           kind: left.kind,
           file: file,
           scope: scope,
@@ -397,20 +397,20 @@ export default function ({ types: t }) {
 
         path.ensureBlock();
 
-        let block = node.body;
+        const block = node.body;
         block.body = nodes.concat(block.body);
       },
 
       CatchClause({ node, scope }, file) {
-        let pattern = node.param;
+        const pattern = node.param;
         if (!t.isPattern(pattern)) return;
 
-        let ref = scope.generateUidIdentifier("ref");
+        const ref = scope.generateUidIdentifier("ref");
         node.param = ref;
 
-        let nodes = [];
+        const nodes = [];
 
-        let destructuring = new DestructuringTransformer({
+        const destructuring = new DestructuringTransformer({
           kind: "let",
           file: file,
           scope: scope,
@@ -422,12 +422,12 @@ export default function ({ types: t }) {
       },
 
       AssignmentExpression(path, file) {
-        let { node, scope } = path;
+        const { node, scope } = path;
         if (!t.isPattern(node.left)) return;
 
-        let nodes = [];
+        const nodes = [];
 
-        let destructuring = new DestructuringTransformer({
+        const destructuring = new DestructuringTransformer({
           operator: node.operator,
           file: file,
           scope: scope,
@@ -457,21 +457,21 @@ export default function ({ types: t }) {
       },
 
       VariableDeclaration(path, file) {
-        let { node, scope, parent } = path;
+        const { node, scope, parent } = path;
         if (t.isForXStatement(parent)) return;
         if (!parent || !path.container) return; // i don't know why this is necessary - TODO
         if (!variableDeclarationHasPattern(node)) return;
 
-        let nodes = [];
+        const nodes = [];
         let declar;
 
         for (let i = 0; i < node.declarations.length; i++) {
           declar = node.declarations[i];
 
-          let patternId = declar.init;
-          let pattern   = declar.id;
+          const patternId = declar.init;
+          const pattern   = declar.id;
 
-          let destructuring = new DestructuringTransformer({
+          const destructuring = new DestructuringTransformer({
             blockHoist: node._blockHoist,
             nodes:      nodes,
             scope:      scope,
diff --git a/packages/babel-plugin-transform-es2015-duplicate-keys/src/index.js b/packages/babel-plugin-transform-es2015-duplicate-keys/src/index.js
index 551e27a4ecd7..f2d7a6990f25 100644
--- a/packages/babel-plugin-transform-es2015-duplicate-keys/src/index.js
+++ b/packages/babel-plugin-transform-es2015-duplicate-keys/src/index.js
@@ -26,27 +26,27 @@ export default function() {
         const alreadySeenGetters = Object.create(null);
         const alreadySeenSetters = Object.create(null);
 
-        for (let prop of plainProps) {
+        for (const prop of plainProps) {
           const name = getName(prop.key);
           let isDuplicate = false;
           switch (prop.kind) {
-          case "get":
-            if (alreadySeenData[name] || alreadySeenGetters[name]) {
-              isDuplicate = true;
-            }
-            alreadySeenGetters[name] = true;
-            break;
-          case "set":
-            if (alreadySeenData[name] || alreadySeenSetters[name]) {
-              isDuplicate = true;
-            }
-            alreadySeenSetters[name] = true;
-            break;
-          default:
-            if (alreadySeenData[name] || alreadySeenGetters[name] || alreadySeenSetters[name]) {
-              isDuplicate = true;
-            }
-            alreadySeenData[name] = true;
+            case "get":
+              if (alreadySeenData[name] || alreadySeenGetters[name]) {
+                isDuplicate = true;
+              }
+              alreadySeenGetters[name] = true;
+              break;
+            case "set":
+              if (alreadySeenData[name] || alreadySeenSetters[name]) {
+                isDuplicate = true;
+              }
+              alreadySeenSetters[name] = true;
+              break;
+            default:
+              if (alreadySeenData[name] || alreadySeenGetters[name] || alreadySeenSetters[name]) {
+                isDuplicate = true;
+              }
+              alreadySeenData[name] = true;
           }
 
           if (isDuplicate) {
diff --git a/packages/babel-plugin-transform-es2015-for-of/src/index.js b/packages/babel-plugin-transform-es2015-for-of/src/index.js
index ca5ab6c09084..760e0e69b00c 100644
--- a/packages/babel-plugin-transform-es2015-for-of/src/index.js
+++ b/packages/babel-plugin-transform-es2015-for-of/src/index.js
@@ -1,11 +1,11 @@
 /* eslint max-len: 0 */
 
 export default function ({ messages, template, types: t }) {
-  let buildForOfArray = template(`
+  const buildForOfArray = template(`
     for (var KEY = 0; KEY < ARR.length; KEY++) BODY;
   `);
 
-  let buildForOfLoose = template(`
+  const buildForOfLoose = template(`
     for (var LOOP_OBJECT = OBJECT,
              IS_ARRAY = Array.isArray(LOOP_OBJECT),
              INDEX = 0,
@@ -22,7 +22,7 @@ export default function ({ messages, template, types: t }) {
     }
   `);
 
-  let buildForOf = template(`
+  const buildForOf = template(`
     var ITERATOR_COMPLETION = true;
     var ITERATOR_HAD_ERROR_KEY = false;
     var ITERATOR_ERROR_KEY = undefined;
@@ -46,19 +46,19 @@ export default function ({ messages, template, types: t }) {
   `);
 
   function _ForOfStatementArray(path) {
-    let { node, scope } = path;
-    let nodes = [];
+    const { node, scope } = path;
+    const nodes = [];
     let right = node.right;
 
     if (!t.isIdentifier(right) || !scope.hasBinding(right.name)) {
-      let uid = scope.generateUidIdentifier("arr");
+      const uid = scope.generateUidIdentifier("arr");
       nodes.push(t.variableDeclaration("var", [
         t.variableDeclarator(uid, right)
       ]));
       right = uid;
     }
 
-    let iterationKey = scope.generateUidIdentifier("i");
+    const iterationKey = scope.generateUidIdentifier("i");
 
     let loop = buildForOfArray({
       BODY: node.body,
@@ -69,9 +69,9 @@ export default function ({ messages, template, types: t }) {
     t.inherits(loop, node);
     t.ensureBlock(loop);
 
-    let iterationValue = t.memberExpression(right, iterationKey, true);
+    const iterationValue = t.memberExpression(right, iterationKey, true);
 
-    let left = node.left;
+    const left = node.left;
     if (t.isVariableDeclaration(left)) {
       left.declarations[0].init = iterationValue;
       loop.body.body.unshift(left);
@@ -103,11 +103,11 @@ export default function ({ messages, template, types: t }) {
         let callback = spec;
         if (state.opts.loose) callback = loose;
 
-        let { node } = path;
-        let build  = callback(path, state);
-        let declar = build.declar;
-        let loop   = build.loop;
-        let block  = loop.body;
+        const { node } = path;
+        const build  = callback(path, state);
+        const declar = build.declar;
+        const loop   = build.loop;
+        const block  = loop.body;
 
         // ensure that it's a block so we can take all its statements
         path.ensureBlock();
@@ -134,9 +134,9 @@ export default function ({ messages, template, types: t }) {
   };
 
   function loose(path, file) {
-    let { node, scope } = path;
+    const { node, scope } = path;
 
-    let left = node.left;
+    const left = node.left;
     let declar, id;
 
     if (t.isIdentifier(left) || t.isPattern(left) || t.isMemberExpression(left)) {
@@ -152,10 +152,10 @@ export default function ({ messages, template, types: t }) {
       throw file.buildCodeFrameError(left, messages.get("unknownForHead", left.type));
     }
 
-    let iteratorKey = scope.generateUidIdentifier("iterator");
-    let isArrayKey  = scope.generateUidIdentifier("isArray");
+    const iteratorKey = scope.generateUidIdentifier("iterator");
+    const isArrayKey  = scope.generateUidIdentifier("isArray");
 
-    let loop = buildForOfLoose({
+    const loop = buildForOfLoose({
       LOOP_OBJECT:  iteratorKey,
       IS_ARRAY:     isArrayKey,
       OBJECT:       node.right,
@@ -179,12 +179,12 @@ export default function ({ messages, template, types: t }) {
   }
 
   function spec(path, file) {
-    let { node, scope, parent } = path;
-    let left = node.left;
+    const { node, scope, parent } = path;
+    const left = node.left;
     let declar;
 
-    let stepKey   = scope.generateUidIdentifier("step");
-    let stepValue = t.memberExpression(stepKey, t.identifier("value"));
+    const stepKey   = scope.generateUidIdentifier("step");
+    const stepValue = t.memberExpression(stepKey, t.identifier("value"));
 
     if (t.isIdentifier(left) || t.isPattern(left) || t.isMemberExpression(left)) {
       // for (i of test), for ({ i } of test)
@@ -200,9 +200,9 @@ export default function ({ messages, template, types: t }) {
 
     //
 
-    let iteratorKey = scope.generateUidIdentifier("iterator");
+    const iteratorKey = scope.generateUidIdentifier("iterator");
 
-    let template = buildForOf({
+    const template = buildForOf({
       ITERATOR_HAD_ERROR_KEY: scope.generateUidIdentifier("didIteratorError"),
       ITERATOR_COMPLETION:    scope.generateUidIdentifier("iteratorNormalCompletion"),
       ITERATOR_ERROR_KEY:     scope.generateUidIdentifier("iteratorError"),
@@ -212,10 +212,10 @@ export default function ({ messages, template, types: t }) {
       BODY:                   null
     });
 
-    let isLabeledParent = t.isLabeledStatement(parent);
+    const isLabeledParent = t.isLabeledStatement(parent);
 
-    let tryBody = template[3].block.body;
-    let loop = tryBody[0];
+    const tryBody = template[3].block.body;
+    const loop = tryBody[0];
 
     if (isLabeledParent) {
       tryBody[0] = t.labeledStatement(parent.label, loop);
diff --git a/packages/babel-plugin-transform-es2015-function-name/src/index.js b/packages/babel-plugin-transform-es2015-function-name/src/index.js
index ab403e6ec5f1..6fdeedd06935 100644
--- a/packages/babel-plugin-transform-es2015-function-name/src/index.js
+++ b/packages/babel-plugin-transform-es2015-function-name/src/index.js
@@ -6,16 +6,16 @@ export default function () {
       "ArrowFunctionExpression|FunctionExpression": {
         exit(path) {
           if (path.key !== "value" && !path.parentPath.isObjectProperty()) {
-            let replacement = nameFunction(path);
+            const replacement = nameFunction(path);
             if (replacement) path.replaceWith(replacement);
           }
         }
       },
 
       ObjectProperty(path) {
-        let value = path.get("value");
+        const value = path.get("value");
         if (value.isFunction()) {
-          let newNode = nameFunction(value);
+          const newNode = nameFunction(value);
           if (newNode) value.replaceWith(newNode);
         }
       }
diff --git a/packages/babel-plugin-transform-es2015-instanceof/src/index.js b/packages/babel-plugin-transform-es2015-instanceof/src/index.js
index d12f4c6fc98c..ff744f9cff64 100644
--- a/packages/babel-plugin-transform-es2015-instanceof/src/index.js
+++ b/packages/babel-plugin-transform-es2015-instanceof/src/index.js
@@ -2,7 +2,7 @@ export default function ({ types: t }) {
   return {
     visitor: {
       BinaryExpression(path) {
-        let { node } = path;
+        const { node } = path;
         if (node.operator === "instanceof") {
           path.replaceWith(t.callExpression(this.addHelper("instanceof"), [node.left, node.right]));
         }
diff --git a/packages/babel-plugin-transform-es2015-modules-amd/src/index.js b/packages/babel-plugin-transform-es2015-modules-amd/src/index.js
index 3c5f9243bc8f..bb9eb42c9614 100644
--- a/packages/babel-plugin-transform-es2015-modules-amd/src/index.js
+++ b/packages/babel-plugin-transform-es2015-modules-amd/src/index.js
@@ -1,10 +1,10 @@
 import template from "babel-template";
 
-let buildDefine = template(`
+const buildDefine = template(`
   define(MODULE_NAME, [SOURCES], FACTORY);
 `);
 
-let buildFactory = template(`
+const buildFactory = template(`
   (function (PARAMS) {
     BODY;
   })
@@ -16,16 +16,16 @@ export default function ({ types: t }) {
     if (!path.get("callee").isIdentifier({ name: "require" })) return false;
     if (path.scope.getBinding("require")) return false;
 
-    let args = path.get("arguments");
+    const args = path.get("arguments");
     if (args.length !== 1) return false;
 
-    let arg = args[0];
+    const arg = args[0];
     if (!arg.isStringLiteral()) return false;
 
     return true;
   }
 
-  let amdVisitor = {
+  const amdVisitor = {
     ReferencedIdentifier({ node, scope }) {
       if (node.name === "exports" && !scope.getBinding("exports")) {
         this.hasExports = true;
@@ -43,13 +43,13 @@ export default function ({ types: t }) {
     },
 
     VariableDeclarator(path) {
-      let id = path.get("id");
+      const id = path.get("id");
       if (!id.isIdentifier()) return;
 
-      let init = path.get("init");
+      const init = path.get("init");
       if (!isValidRequireCall(init)) return;
 
-      let source = init.node.arguments[0];
+      const source = init.node.arguments[0];
       this.sourceNames[source.value] = true;
       this.sources.push([id.node, source]);
 
@@ -80,7 +80,7 @@ export default function ({ types: t }) {
 
           path.traverse(amdVisitor, this);
 
-          let params = this.sources.map((source) => source[0]);
+          const params = this.sources.map((source) => source[0]);
           let sources = this.sources.map((source) => source[1]);
 
           sources = sources.concat(this.bareSources.filter((str) => {
@@ -100,8 +100,8 @@ export default function ({ types: t }) {
             params.unshift(t.identifier("module"));
           }
 
-          let { node } = path;
-          let factory = buildFactory({
+          const { node } = path;
+          const factory = buildFactory({
             PARAMS: params,
             BODY: node.body
           });
diff --git a/packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js b/packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js
index 8d3351bfa92e..a7285da44645 100644
--- a/packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js
+++ b/packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js
@@ -4,17 +4,17 @@ import { basename, extname } from "path";
 import template from "babel-template";
 import * as t from "babel-types";
 
-let buildRequire = template(`
+const buildRequire = template(`
   require($0);
 `);
 
-let buildExportsModuleDeclaration = template(`
+const buildExportsModuleDeclaration = template(`
   Object.defineProperty(exports, "__esModule", {
     value: true
   });
 `);
 
-let buildExportsFrom = template(`
+const buildExportsFrom = template(`
   Object.defineProperty(exports, $0, {
     enumerable: true,
     get: function () {
@@ -23,15 +23,15 @@ let buildExportsFrom = template(`
   });
 `);
 
-let buildLooseExportsModuleDeclaration = template(`
+const buildLooseExportsModuleDeclaration = template(`
   exports.__esModule = true;
 `);
 
-let buildExportsAssignment = template(`
+const buildExportsAssignment = template(`
   exports.$0 = $1;
 `);
 
-let buildExportAll = template(`
+const buildExportAll = template(`
   Object.keys(OBJECT).forEach(function (key) {
     if (key === "default" || key === "__esModule") return;
     Object.defineProperty(exports, key, {
@@ -46,12 +46,12 @@ let buildExportAll = template(`
 const THIS_BREAK_KEYS = ["FunctionExpression", "FunctionDeclaration", "ClassProperty", "ClassMethod", "ObjectMethod"];
 
 export default function () {
-  let REASSIGN_REMAP_SKIP = Symbol();
+  const REASSIGN_REMAP_SKIP = Symbol();
 
-  let reassignmentVisitor = {
+  const reassignmentVisitor = {
     ReferencedIdentifier(path) {
-      let name = path.node.name;
-      let remap = this.remaps[name];
+      const name = path.node.name;
+      const remap = this.remaps[name];
       if (!remap) return;
 
       // redeclared in this scope
@@ -72,11 +72,11 @@ export default function () {
       let node = path.node;
       if (node[REASSIGN_REMAP_SKIP]) return;
 
-      let left = path.get("left");
+      const left = path.get("left");
       if (!left.isIdentifier()) return;
 
-      let name = left.node.name;
-      let exports = this.exports[name];
+      const name = left.node.name;
+      const exports = this.exports[name];
       if (!exports) return;
 
       // redeclared in this scope
@@ -84,7 +84,7 @@ export default function () {
 
       node[REASSIGN_REMAP_SKIP] = true;
 
-      for (let reid of exports) {
+      for (const reid of exports) {
         node = buildExportsAssignment(reid, node).expression;
       }
 
@@ -93,17 +93,17 @@ export default function () {
     },
 
     UpdateExpression(path) {
-      let arg = path.get("argument");
+      const arg = path.get("argument");
       if (!arg.isIdentifier()) return;
 
-      let name = arg.node.name;
-      let exports = this.exports[name];
+      const name = arg.node.name;
+      const exports = this.exports[name];
       if (!exports) return;
 
       // redeclared in this scope
       if (this.scope.getBinding(name) !== path.scope.getBinding(name)) return;
 
-      let node = t.assignmentExpression(path.node.operator[0] + "=", arg.node, t.numericLiteral(1));
+      const node = t.assignmentExpression(path.node.operator[0] + "=", arg.node, t.numericLiteral(1));
 
       if ((path.parentPath.isExpressionStatement() && !path.isCompletionRecord()) || path.node.prefix) {
         path.replaceWith(node);
@@ -111,7 +111,7 @@ export default function () {
         return;
       }
 
-      let nodes = [];
+      const nodes = [];
       nodes.push(node);
 
       let operator;
@@ -149,9 +149,9 @@ export default function () {
         exit(path) {
           this.ranCommonJS = true;
 
-          let strict = !!this.opts.strict;
+          const strict = !!this.opts.strict;
 
-          let { scope } = path;
+          const { scope } = path;
 
           // rename these commonjs variables if they're declared in the file
           scope.rename("module");
@@ -161,24 +161,24 @@ export default function () {
           let hasExports = false;
           let hasImports = false;
 
-          let body: Array<Object> = path.get("body");
-          let imports = Object.create(null);
-          let exports = Object.create(null);
+          const body: Array<Object> = path.get("body");
+          const imports = Object.create(null);
+          const exports = Object.create(null);
 
-          let nonHoistedExportNames = Object.create(null);
+          const nonHoistedExportNames = Object.create(null);
 
-          let topNodes = [];
-          let remaps = Object.create(null);
+          const topNodes = [];
+          const remaps = Object.create(null);
 
-          let requires = Object.create(null);
+          const requires = Object.create(null);
 
           function addRequire(source, blockHoist) {
-            let cached = requires[source];
+            const cached = requires[source];
             if (cached) return cached;
 
-            let ref = path.scope.generateUidIdentifier(basename(source, extname(source)));
+            const ref = path.scope.generateUidIdentifier(basename(source, extname(source)));
 
-            let varDecl = t.variableDeclaration("var", [
+            const varDecl = t.variableDeclaration("var", [
               t.variableDeclarator(ref, buildRequire(
                 t.stringLiteral(source)
               ).expression)
@@ -200,17 +200,17 @@ export default function () {
           }
 
           function addTo(obj, key, arr) {
-            let existing = obj[key] || [];
+            const existing = obj[key] || [];
             obj[key] = existing.concat(arr);
           }
 
-          for (let path of body) {
+          for (const path of body) {
             if (path.isExportDeclaration()) {
               hasExports = true;
 
-              let specifiers = [].concat(path.get("declaration"), path.get("specifiers"));
-              for (let specifier of specifiers) {
-                let ids = specifier.getBindingIdentifiers();
+              const specifiers = [].concat(path.get("declaration"), path.get("specifiers"));
+              for (const specifier of specifiers) {
+                const ids = specifier.getBindingIdentifiers();
                 if (ids.__esModule) {
                   throw specifier.buildCodeFrameError("Illegal export \"__esModule\"");
                 }
@@ -220,8 +220,8 @@ export default function () {
             if (path.isImportDeclaration()) {
               hasImports = true;
 
-              let key = path.node.source.value;
-              let importsEntry = imports[key] || {
+              const key = path.node.source.value;
+              const importsEntry = imports[key] || {
                 specifiers: [],
                 maxBlockHoist: 0,
                 loc: path.node.loc,
@@ -240,10 +240,10 @@ export default function () {
 
               path.remove();
             } else if (path.isExportDefaultDeclaration()) {
-              let declaration = path.get("declaration");
+              const declaration = path.get("declaration");
               if (declaration.isFunctionDeclaration()) {
-                let id = declaration.node.id;
-                let defNode = t.identifier("default");
+                const id = declaration.node.id;
+                const defNode = t.identifier("default");
                 if (id) {
                   addTo(exports, id.name, defNode);
                   topNodes.push(buildExportsAssignment(defNode, id));
@@ -253,8 +253,8 @@ export default function () {
                   path.remove();
                 }
               } else if (declaration.isClassDeclaration()) {
-                let id = declaration.node.id;
-                let defNode = t.identifier("default");
+                const id = declaration.node.id;
+                const defNode = t.identifier("default");
                 if (id) {
                   addTo(exports, id.name, defNode);
                   path.replaceWithMultiple([
@@ -278,15 +278,15 @@ export default function () {
                 path.parentPath.requeue(path.get("expression.left"));
               }
             } else if (path.isExportNamedDeclaration()) {
-              let declaration = path.get("declaration");
+              const declaration = path.get("declaration");
               if (declaration.node) {
                 if (declaration.isFunctionDeclaration()) {
-                  let id = declaration.node.id;
+                  const id = declaration.node.id;
                   addTo(exports, id.name, id);
                   topNodes.push(buildExportsAssignment(id, id));
                   path.replaceWith(declaration.node);
                 } else if (declaration.isClassDeclaration()) {
-                  let id = declaration.node.id;
+                  const id = declaration.node.id;
                   addTo(exports, id.name, id);
                   path.replaceWithMultiple([
                     declaration.node,
@@ -294,11 +294,11 @@ export default function () {
                   ]);
                   nonHoistedExportNames[id.name] = true;
                 } else if (declaration.isVariableDeclaration()) {
-                  let declarators = declaration.get("declarations");
-                  for (let decl of declarators) {
-                    let id = decl.get("id");
+                  const declarators = declaration.get("declarations");
+                  for (const decl of declarators) {
+                    const id = decl.get("id");
 
-                    let init = decl.get("init");
+                    const init = decl.get("init");
                     if (!init.node) init.replaceWith(t.identifier("undefined"));
 
                     if (id.isIdentifier()) {
@@ -314,13 +314,13 @@ export default function () {
                 continue;
               }
 
-              let specifiers = path.get("specifiers");
-              let nodes = [];
-              let source = path.node.source;
+              const specifiers = path.get("specifiers");
+              const nodes = [];
+              const source = path.node.source;
               if (source) {
-                let ref = addRequire(source.value, path.node._blockHoist);
+                const ref = addRequire(source.value, path.node._blockHoist);
 
-                for (let specifier of specifiers) {
+                for (const specifier of specifiers) {
                   if (specifier.isExportNamespaceSpecifier()) {
                     // todo
                   } else if (specifier.isExportDefaultSpecifier()) {
@@ -335,7 +335,7 @@ export default function () {
                   }
                 }
               } else {
-                for (let specifier of specifiers) {
+                for (const specifier of specifiers) {
                   if (specifier.isExportSpecifier()) {
                     addTo(exports, specifier.node.local.name, specifier.node.exported);
                     nonHoistedExportNames[specifier.node.exported.name] = true;
@@ -345,7 +345,7 @@ export default function () {
               }
               path.replaceWithMultiple(nodes);
             } else if (path.isExportAllDeclaration()) {
-              let exportNode = buildExportAll({
+              const exportNode = buildExportAll({
                 OBJECT: addRequire(path.node.source.value, path.node._blockHoist)
               });
               exportNode.loc = path.node.loc;
@@ -354,15 +354,15 @@ export default function () {
             }
           }
 
-          for (let source in imports) {
-            let {specifiers, maxBlockHoist} = imports[source];
+          for (const source in imports) {
+            const {specifiers, maxBlockHoist} = imports[source];
             if (specifiers.length) {
-              let uid = addRequire(source, maxBlockHoist);
+              const uid = addRequire(source, maxBlockHoist);
 
               let wildcard;
 
               for (let i = 0; i < specifiers.length; i++) {
-                let specifier = specifiers[i];
+                const specifier = specifiers[i];
                 if (t.isImportNamespaceSpecifier(specifier)) {
                   if (strict) {
                     remaps[specifier.local.name] = uid;
@@ -389,7 +389,7 @@ export default function () {
                 }
               }
 
-              for (let specifier of specifiers) {
+              for (const specifier of specifiers) {
                 if (t.isImportSpecifier(specifier)) {
                   let target = uid;
                   if (specifier.imported.name === "default") {
@@ -419,7 +419,7 @@ export default function () {
               }
             } else {
               // bare import
-              let requireNode = buildRequire(t.stringLiteral(source));
+              const requireNode = buildRequire(t.stringLiteral(source));
               requireNode.loc = imports[source].loc;
               topNodes.push(requireNode);
             }
@@ -428,7 +428,7 @@ export default function () {
           if (hasImports && Object.keys(nonHoistedExportNames).length) {
             let hoistedExportsNode = t.identifier("undefined");
 
-            for (let name in nonHoistedExportNames) {
+            for (const name in nonHoistedExportNames) {
               hoistedExportsNode = buildExportsAssignment(t.identifier(name), hoistedExportsNode).expression;
             }
 
diff --git a/packages/babel-plugin-transform-es2015-modules-commonjs/test/esmodule-flag.js b/packages/babel-plugin-transform-es2015-modules-commonjs/test/esmodule-flag.js
index beee1e9cd3ee..2ab422c89b68 100644
--- a/packages/babel-plugin-transform-es2015-modules-commonjs/test/esmodule-flag.js
+++ b/packages/babel-plugin-transform-es2015-modules-commonjs/test/esmodule-flag.js
@@ -1,14 +1,14 @@
-let assert = require("assert");
-let babel = require("babel-core");
-let vm = require("vm");
+const assert = require("assert");
+const babel = require("babel-core");
+const vm = require("vm");
 
 test("Re-export doesn't overwrite __esModule flag", function () {
   let code = "export * from \"./dep\";";
-  let depStub = {
+  const depStub = {
     __esModule: false,
   };
 
-  let context = {
+  const context = {
     module: {
       exports: {}
     },
diff --git a/packages/babel-plugin-transform-es2015-modules-systemjs/src/index.js b/packages/babel-plugin-transform-es2015-modules-systemjs/src/index.js
index 93091bac29de..c79baab12bd1 100644
--- a/packages/babel-plugin-transform-es2015-modules-systemjs/src/index.js
+++ b/packages/babel-plugin-transform-es2015-modules-systemjs/src/index.js
@@ -3,7 +3,7 @@
 import hoistVariables from "babel-helper-hoist-variables";
 import template from "babel-template";
 
-let buildTemplate = template(`
+const buildTemplate = template(`
   SYSTEM_REGISTER(MODULE_NAME, [SOURCES], function (EXPORT_IDENTIFIER, CONTEXT_IDENTIFIER) {
     "use strict";
     BEFORE_BODY;
@@ -16,7 +16,7 @@ let buildTemplate = template(`
   });
 `);
 
-let buildExportAll = template(`
+const buildExportAll = template(`
   for (var KEY in TARGET) {
     if (KEY !== "default" && KEY !== "__esModule") EXPORT_OBJ[KEY] = TARGET[KEY];
   }
@@ -26,22 +26,22 @@ let buildExportAll = template(`
 const TYPE_IMPORT = "Import";
 
 export default function ({ types: t }) {
-  let IGNORE_REASSIGNMENT_SYMBOL = Symbol();
+  const IGNORE_REASSIGNMENT_SYMBOL = Symbol();
 
-  let reassignmentVisitor = {
+  const reassignmentVisitor = {
     "AssignmentExpression|UpdateExpression"(path) {
       if (path.node[IGNORE_REASSIGNMENT_SYMBOL]) return;
       path.node[IGNORE_REASSIGNMENT_SYMBOL] = true;
 
-      let arg = path.get(path.isAssignmentExpression() ? "left" : "argument");
+      const arg = path.get(path.isAssignmentExpression() ? "left" : "argument");
       if (!arg.isIdentifier()) return;
 
-      let name = arg.node.name;
+      const name = arg.node.name;
 
       // redeclared in this scope
       if (this.scope.getBinding(name) !== path.scope.getBinding(name)) return;
 
-      let exportedNames = this.exports[name];
+      const exportedNames = this.exports[name];
       if (!exportedNames) return;
 
       let node = path.node;
@@ -59,7 +59,7 @@ export default function ({ types: t }) {
           isPostUpdateExpression = false;
       }
 
-      for (let exportedName of exportedNames) {
+      for (const exportedName of exportedNames) {
         node = this.buildCall(exportedName, node).expression;
       }
 
@@ -75,7 +75,7 @@ export default function ({ types: t }) {
 
       CallExpression(path, state) {
         if (path.node.callee.type === TYPE_IMPORT) {
-          let contextIdent = state.contextIdent;
+          const contextIdent = state.contextIdent;
           path.replaceWith(t.callExpression(t.memberExpression(contextIdent, t.identifier("import")), path.node.arguments));
         }
       },
@@ -91,17 +91,17 @@ export default function ({ types: t }) {
           state.contextIdent = path.scope.generateUidIdentifier("context");
         },
         exit(path, state) {
-          let exportIdent = path.scope.generateUidIdentifier("export");
-          let contextIdent = state.contextIdent;
+          const exportIdent = path.scope.generateUidIdentifier("export");
+          const contextIdent = state.contextIdent;
 
-          let exportNames = Object.create(null);
-          let modules = [];
+          const exportNames = Object.create(null);
+          const modules = [];
 
           let beforeBody = [];
-          let setters = [];
-          let sources = [];
-          let variableIds = [];
-          let removedPaths = [];
+          const setters = [];
+          const sources = [];
+          const variableIds = [];
+          const removedPaths = [];
 
           function addExportName(key, val) {
             exportNames[key] = exportNames[key] || [];
@@ -127,7 +127,7 @@ export default function ({ types: t }) {
             );
           }
 
-          let body: Array<Object> = path.get("body");
+          const body: Array<Object> = path.get("body");
 
           let canHoist = true;
           for (let path of body) {
@@ -138,14 +138,14 @@ export default function ({ types: t }) {
             }
           }
 
-          for (let path of body) {
+          for (const path of body) {
             if (canHoist && path.isFunctionDeclaration()) {
               beforeBody.push(path.node);
               removedPaths.push(path);
             } else if (path.isImportDeclaration()) {
-              let source = path.node.source.value;
+              const source = path.node.source.value;
               pushModule(source, "imports", path.node.specifiers);
-              for (let name in path.getBindingIdentifiers()) {
+              for (const name in path.getBindingIdentifiers()) {
                 path.scope.removeBinding(name);
                 variableIds.push(t.identifier(name));
               }
@@ -154,10 +154,10 @@ export default function ({ types: t }) {
               pushModule(path.node.source.value, "exports", path.node);
               path.remove();
             } else if (path.isExportDefaultDeclaration()) {
-              let declar = path.get("declaration");
+              const declar = path.get("declaration");
               if (declar.isClassDeclaration() || declar.isFunctionDeclaration()) {
-                let id = declar.node.id;
-                let nodes = [];
+                const id = declar.node.id;
+                const nodes = [];
 
                 if (id) {
                   nodes.push(declar.node);
@@ -177,16 +177,16 @@ export default function ({ types: t }) {
                 path.replaceWith(buildExportCall("default", declar.node));
               }
             } else if (path.isExportNamedDeclaration()) {
-              let declar = path.get("declaration");
+              const declar = path.get("declaration");
 
               if (declar.node) {
                 path.replaceWith(declar);
 
-                let nodes = [];
+                const nodes = [];
                 let bindingIdentifiers;
                 if (path.isFunction()) {
-                  let node = declar.node;
-                  let name = node.id.name;
+                  const node = declar.node;
+                  const name = node.id.name;
                   if (canHoist) {
                     addExportName(name, name);
                     beforeBody.push(node);
@@ -198,21 +198,21 @@ export default function ({ types: t }) {
                 } else {
                   bindingIdentifiers = declar.getBindingIdentifiers();
                 }
-                for (let name in bindingIdentifiers) {
+                for (const name in bindingIdentifiers) {
                   addExportName(name, name);
                   nodes.push(buildExportCall(name, t.identifier(name)));
                 }
                 path.insertAfter(nodes);
               } else {
-                let specifiers = path.node.specifiers;
+                const specifiers = path.node.specifiers;
                 if (specifiers && specifiers.length) {
                   if (path.node.source) {
                     pushModule(path.node.source.value, "exports", specifiers);
                     path.remove();
                   } else {
-                    let nodes = [];
+                    const nodes = [];
 
-                    for (let specifier of specifiers) {
+                    for (const specifier of specifiers) {
                       nodes.push(buildExportCall(specifier.exported.name, specifier.local));
                       addExportName(specifier.local.name, specifier.exported.name);
                     }
@@ -225,8 +225,8 @@ export default function ({ types: t }) {
           }
 
           modules.forEach(function (specifiers) {
-            let setterBody = [];
-            let target = path.scope.generateUidIdentifier(specifiers.key);
+            const setterBody = [];
+            const target = path.scope.generateUidIdentifier(specifiers.key);
 
             for (let specifier of specifiers.imports) {
               if (t.isImportNamespaceSpecifier(specifier)) {
@@ -241,13 +241,13 @@ export default function ({ types: t }) {
             }
 
             if (specifiers.exports.length) {
-              let exportObjRef = path.scope.generateUidIdentifier("exportObj");
+              const exportObjRef = path.scope.generateUidIdentifier("exportObj");
 
               setterBody.push(t.variableDeclaration("var", [
                 t.variableDeclarator(exportObjRef, t.objectExpression([]))
               ]));
 
-              for (let node of specifiers.exports) {
+              for (const node of specifiers.exports) {
                 if (t.isExportAllDeclaration(node)) {
                   setterBody.push(buildExportAll({
                     KEY: path.scope.generateUidIdentifier("key"),
@@ -288,7 +288,7 @@ export default function ({ types: t }) {
             scope: path.scope
           });
 
-          for (let path of removedPaths) {
+          for (const path of removedPaths) {
             path.remove();
           }
 
diff --git a/packages/babel-plugin-transform-es2015-modules-umd/src/index.js b/packages/babel-plugin-transform-es2015-modules-umd/src/index.js
index 44b88ee55d65..b13ab6af37dd 100644
--- a/packages/babel-plugin-transform-es2015-modules-umd/src/index.js
+++ b/packages/babel-plugin-transform-es2015-modules-umd/src/index.js
@@ -3,18 +3,18 @@
 import { basename, extname } from "path";
 import template from "babel-template";
 
-let buildPrerequisiteAssignment = template(`
+const buildPrerequisiteAssignment = template(`
   GLOBAL_REFERENCE = GLOBAL_REFERENCE || {}
 `);
 
-let buildGlobalExport = template(`
+const buildGlobalExport = template(`
   var mod = { exports: {} };
   factory(BROWSER_ARGUMENTS);
   PREREQUISITE_ASSIGNMENTS
   GLOBAL_TO_ASSIGN = mod.exports;
 `);
 
-let buildWrapper = template(`
+const buildWrapper = template(`
   (function (global, factory) {
     if (typeof define === "function" && define.amd) {
       define(MODULE_NAME, AMD_ARGUMENTS, factory);
@@ -30,11 +30,11 @@ export default function ({ types: t }) {
   function isValidDefine(path) {
     if (!path.isExpressionStatement()) return;
 
-    let expr = path.get("expression");
+    const expr = path.get("expression");
     if (!expr.isCallExpression()) return false;
     if (!expr.get("callee").isIdentifier({ name: "define" })) return false;
 
-    let args = expr.get("arguments");
+    const args = expr.get("arguments");
     if (args.length === 3 && !args.shift().isStringLiteral()) return false;
     if (args.length !== 2) return false;
     if (!args.shift().isArrayExpression()) return false;
@@ -49,18 +49,18 @@ export default function ({ types: t }) {
     visitor: {
       Program: {
         exit(path, state) {
-          let last = path.get("body").pop();
+          const last = path.get("body").pop();
           if (!isValidDefine(last)) return;
 
-          let call = last.node.expression;
-          let args = call.arguments;
+          const call = last.node.expression;
+          const args = call.arguments;
 
-          let moduleName = args.length === 3 ? args.shift() : null;
-          let amdArgs = call.arguments[0];
-          let func = call.arguments[1];
-          let browserGlobals = state.opts.globals || {};
+          const moduleName = args.length === 3 ? args.shift() : null;
+          const amdArgs = call.arguments[0];
+          const func = call.arguments[1];
+          const browserGlobals = state.opts.globals || {};
 
-          let commonArgs = amdArgs.elements.map((arg) => {
+          const commonArgs = amdArgs.elements.map((arg) => {
             if (arg.value === "module" || arg.value === "exports") {
               return t.identifier(arg.value);
             } else {
@@ -68,7 +68,7 @@ export default function ({ types: t }) {
             }
           });
 
-          let browserArgs = amdArgs.elements.map((arg) => {
+          const browserArgs = amdArgs.elements.map((arg) => {
             if (arg.value === "module") {
               return t.identifier("mod");
             } else if (arg.value === "exports") {
@@ -77,7 +77,7 @@ export default function ({ types: t }) {
               let memberExpression;
 
               if (state.opts.exactGlobals) {
-                let globalRef = browserGlobals[arg.value];
+                const globalRef = browserGlobals[arg.value];
                 if (globalRef) {
                   memberExpression = globalRef.split(".").reduce(
                     (accum, curr) => t.memberExpression(accum, t.identifier(curr)), t.identifier("global")
@@ -88,8 +88,8 @@ export default function ({ types: t }) {
                   );
                 }
               } else {
-                let requireName = basename(arg.value, extname(arg.value));
-                let globalName = browserGlobals[requireName] || requireName;
+                const requireName = basename(arg.value, extname(arg.value));
+                const globalName = browserGlobals[requireName] || requireName;
                 memberExpression = t.memberExpression(
                   t.identifier("global"), t.identifier(t.toIdentifier(globalName))
                 );
@@ -99,19 +99,19 @@ export default function ({ types: t }) {
             }
           });
 
-          let moduleNameOrBasename = moduleName ? moduleName.value : this.file.opts.basename;
+          const moduleNameOrBasename = moduleName ? moduleName.value : this.file.opts.basename;
           let globalToAssign = t.memberExpression(
             t.identifier("global"), t.identifier(t.toIdentifier(moduleNameOrBasename))
           );
           let prerequisiteAssignments = null;
 
           if (state.opts.exactGlobals) {
-            let globalName = browserGlobals[moduleNameOrBasename];
+            const globalName = browserGlobals[moduleNameOrBasename];
 
             if (globalName) {
               prerequisiteAssignments = [];
 
-              let members = globalName.split(".");
+              const members = globalName.split(".");
               globalToAssign = members.slice(1).reduce((accum, curr) => {
                 prerequisiteAssignments.push(buildPrerequisiteAssignment({ GLOBAL_REFERENCE: accum }));
                 return t.memberExpression(accum, t.identifier(curr));
@@ -119,7 +119,7 @@ export default function ({ types: t }) {
             }
           }
 
-          let globalExport = buildGlobalExport({
+          const globalExport = buildGlobalExport({
             BROWSER_ARGUMENTS: browserArgs,
             PREREQUISITE_ASSIGNMENTS: prerequisiteAssignments,
             GLOBAL_TO_ASSIGN: globalToAssign
diff --git a/packages/babel-plugin-transform-es2015-object-super/src/index.js b/packages/babel-plugin-transform-es2015-object-super/src/index.js
index 95098fda8a53..5e633fe44728 100644
--- a/packages/babel-plugin-transform-es2015-object-super/src/index.js
+++ b/packages/babel-plugin-transform-es2015-object-super/src/index.js
@@ -2,7 +2,7 @@ import ReplaceSupers from "babel-helper-replace-supers";
 
 export default function ({ types: t }) {
   function Property(path, node, scope, getObjectRef, file) {
-    let replaceSupers = new ReplaceSupers({
+    const replaceSupers = new ReplaceSupers({
       getObjectRef: getObjectRef,
       methodNode:   node,
       methodPath:   path,
@@ -14,12 +14,12 @@ export default function ({ types: t }) {
     replaceSupers.replace();
   }
 
-  let CONTAINS_SUPER = Symbol();
+  const CONTAINS_SUPER = Symbol();
 
   return {
     visitor: {
       Super(path) {
-        let parentObj = path.findParent((path) => path.isObjectExpression());
+        const parentObj = path.findParent((path) => path.isObjectExpression());
         if (parentObj) parentObj.node[CONTAINS_SUPER] = true;
       },
 
@@ -28,9 +28,9 @@ export default function ({ types: t }) {
           if (!path.node[CONTAINS_SUPER]) return;
 
           let objectRef;
-          let getObjectRef = () => objectRef = objectRef || path.scope.generateUidIdentifier("obj");
+          const getObjectRef = () => objectRef = objectRef || path.scope.generateUidIdentifier("obj");
 
-          let propPaths: Array = path.get("properties");
+          const propPaths: Array = path.get("properties");
           for (let propPath of propPaths) {
             if (propPath.isObjectProperty()) propPath = propPath.get("value");
             Property(propPath, propPath.node, path.scope, getObjectRef, file);
diff --git a/packages/babel-plugin-transform-es2015-parameters/src/default.js b/packages/babel-plugin-transform-es2015-parameters/src/default.js
index 445000fadba6..f123ed195ef8 100644
--- a/packages/babel-plugin-transform-es2015-parameters/src/default.js
+++ b/packages/babel-plugin-transform-es2015-parameters/src/default.js
@@ -5,7 +5,7 @@ import callDelegate from "babel-helper-call-delegate";
 import template from "babel-template";
 import * as t from "babel-types";
 
-let buildDefaultParam = template(`
+const buildDefaultParam = template(`
   let VARIABLE_NAME =
     ARGUMENTS.length > ARGUMENT_KEY && ARGUMENTS[ARGUMENT_KEY] !== undefined ?
       ARGUMENTS[ARGUMENT_KEY]
@@ -13,12 +13,12 @@ let buildDefaultParam = template(`
       DEFAULT_VALUE;
 `);
 
-let buildCutOff = template(`
+const buildCutOff = template(`
   let $0 = $1[$2];
 `);
 
 function hasDefaults(node) {
-  for (let param of (node.params: Array<Object>)) {
+  for (const param of (node.params: Array<Object>)) {
     if (!t.isIdentifier(param)) return true;
   }
   return false;
@@ -30,7 +30,7 @@ function isSafeBinding(scope, node) {
   return kind === "param" || kind === "local";
 }
 
-let iifeVisitor = {
+const iifeVisitor = {
   ReferencedIdentifier(path, state) {
     const { scope, node } = path;
     if (node.name === "eval" || !isSafeBinding(scope, node)) {
@@ -45,23 +45,23 @@ let iifeVisitor = {
   }
 };
 
-export let visitor = {
+export const visitor = {
   Function(path) {
-    let { node, scope } = path;
+    const { node, scope } = path;
     if (!hasDefaults(node)) return;
 
     // ensure it's a block, useful for arrow functions
     path.ensureBlock();
 
-    let state = {
+    const state = {
       iife: false,
       scope: scope
     };
 
-    let body = [];
+    const body = [];
 
     //
-    let argsIdentifier = t.identifier("arguments");
+    const argsIdentifier = t.identifier("arguments");
     argsIdentifier._shadowedFunctionLiteral = path;
 
     // push a default parameter definition
@@ -77,12 +77,12 @@ export let visitor = {
     }
 
     //
-    let lastNonDefaultParam = getFunctionArity(node);
+    const lastNonDefaultParam = getFunctionArity(node);
 
     //
-    let params = path.get("params");
+    const params = path.get("params");
     for (let i = 0; i < params.length; i++) {
-      let param = params[i];
+      const param = params[i];
 
       if (!param.isAssignmentPattern()) {
         if (!state.iife && !param.isIdentifier()) {
@@ -92,12 +92,12 @@ export let visitor = {
         continue;
       }
 
-      let left  = param.get("left");
-      let right = param.get("right");
+      const left  = param.get("left");
+      const right = param.get("right");
 
       //
       if (i >= lastNonDefaultParam || left.isPattern()) {
-        let placeholder = scope.generateUidIdentifier("x");
+        const placeholder = scope.generateUidIdentifier("x");
         placeholder._isDefaultPlaceholder = true;
         node.params[i] = placeholder;
       } else {
@@ -119,10 +119,10 @@ export let visitor = {
 
     // add declarations for trailing parameters
     for (let i = lastNonDefaultParam + 1; i < node.params.length; i++) {
-      let param = node.params[i];
+      const param = node.params[i];
       if (param._isDefaultPlaceholder) continue;
 
-      let declar = buildCutOff(param, argsIdentifier, t.numericLiteral(i));
+      const declar = buildCutOff(param, argsIdentifier, t.numericLiteral(i));
       declar._blockHoist = node.params.length - i;
       body.push(declar);
     }
diff --git a/packages/babel-plugin-transform-es2015-parameters/src/destructuring.js b/packages/babel-plugin-transform-es2015-parameters/src/destructuring.js
index e3a7e10494ac..495ac0605092 100644
--- a/packages/babel-plugin-transform-es2015-parameters/src/destructuring.js
+++ b/packages/babel-plugin-transform-es2015-parameters/src/destructuring.js
@@ -1,20 +1,20 @@
 import * as t from "babel-types";
 
-export let visitor = {
+export const visitor = {
   Function(path) {
-    let params: Array = path.get("params");
+    const params: Array = path.get("params");
 
     // If there's a rest param, no need to loop through it. Also, we need to
     // hoist one more level to get `declar` at the right spot.
-    let hoistTweak = t.isRestElement(params[params.length - 1]) ? 1 : 0;
-    let outputParamsLength = params.length - hoistTweak;
+    const hoistTweak = t.isRestElement(params[params.length - 1]) ? 1 : 0;
+    const outputParamsLength = params.length - hoistTweak;
 
     for (let i = 0; i < outputParamsLength; i++) {
-      let param = params[i];
+      const param = params[i];
       if (param.isArrayPattern() || param.isObjectPattern()) {
-        let uid = path.scope.generateUidIdentifier("ref");
+        const uid = path.scope.generateUidIdentifier("ref");
 
-        let declar = t.variableDeclaration("let", [
+        const declar = t.variableDeclaration("let", [
           t.variableDeclarator(param.node, uid)
         ]);
         declar._blockHoist = outputParamsLength - i;
diff --git a/packages/babel-plugin-transform-es2015-parameters/src/index.js b/packages/babel-plugin-transform-es2015-parameters/src/index.js
index 6279147062ba..d59d21b3bf60 100644
--- a/packages/babel-plugin-transform-es2015-parameters/src/index.js
+++ b/packages/babel-plugin-transform-es2015-parameters/src/index.js
@@ -10,8 +10,8 @@ export default function () {
     visitor: visitors.merge([{
       ArrowFunctionExpression(path) {
         // default/rest visitors require access to `arguments`
-        let params: Array<NodePath> = path.get("params");
-        for (let param of params) {
+        const params: Array<NodePath> = path.get("params");
+        for (const param of params) {
           if (param.isRestElement() || param.isAssignmentPattern()) {
             path.arrowFunctionToShadowed();
             break;
diff --git a/packages/babel-plugin-transform-es2015-parameters/src/rest.js b/packages/babel-plugin-transform-es2015-parameters/src/rest.js
index bda77d8c11e1..3ae51ed5488b 100644
--- a/packages/babel-plugin-transform-es2015-parameters/src/rest.js
+++ b/packages/babel-plugin-transform-es2015-parameters/src/rest.js
@@ -3,7 +3,7 @@
 import template from "babel-template";
 import * as t from "babel-types";
 
-let buildRest = template(`
+const buildRest = template(`
   for (var LEN = ARGUMENTS.length,
            ARRAY = Array(ARRAY_LEN),
            KEY = START;
@@ -13,19 +13,19 @@ let buildRest = template(`
   }
 `);
 
-let restIndex = template(`
+const restIndex = template(`
   ARGUMENTS.length <= INDEX ? undefined : ARGUMENTS[INDEX]
 `);
 
-let restIndexImpure = template(`
+const restIndexImpure = template(`
   REF = INDEX, ARGUMENTS.length <= REF ? undefined : ARGUMENTS[REF]
 `);
 
-let restLength = template(`
+const restLength = template(`
   ARGUMENTS.length <= OFFSET ? 0 : ARGUMENTS.length - OFFSET
 `);
 
-let memberExpressionOptimisationVisitor = {
+const memberExpressionOptimisationVisitor = {
   Scope(path, state) {
     // check if this scope has a local binding that will shadow the rest parameter
     if (!path.scope.bindingIdentifierEquals(state.name, state.outerBinding)) {
@@ -43,7 +43,7 @@ let memberExpressionOptimisationVisitor = {
   "Function|ClassProperty": function (path, state) {
     // Detect whether any reference to rest is contained in nested functions to
     // determine if deopt is necessary.
-    let oldNoOptimise = state.noOptimise;
+    const oldNoOptimise = state.noOptimise;
     state.noOptimise = true;
     path.traverse(memberExpressionOptimisationVisitor, state);
     state.noOptimise = oldNoOptimise;
@@ -54,7 +54,7 @@ let memberExpressionOptimisationVisitor = {
   },
 
   ReferencedIdentifier(path, state) {
-    let { node } = path;
+    const { node } = path;
 
     // we can't guarantee the purity of arguments
     if (node.name === "arguments") {
@@ -67,7 +67,7 @@ let memberExpressionOptimisationVisitor = {
     if (state.noOptimise) {
       state.deopted = true;
     } else {
-      let {parentPath} = path;
+      const {parentPath} = path;
 
       // Is this identifier the right hand side of a default parameter?
       if (parentPath.listKey === "params" && parentPath.key < state.offset) {
@@ -77,9 +77,9 @@ let memberExpressionOptimisationVisitor = {
       // ex: `args[0]`
       // ex: `args.whatever`
       if (parentPath.isMemberExpression({ object: node })) {
-        let grandparentPath = parentPath.parentPath;
+        const grandparentPath = parentPath.parentPath;
 
-        let argsOptEligible = !state.deopted && !(
+        const argsOptEligible = !state.deopted && !(
           // ex: `args[0] = "whatever"`
           (
             grandparentPath.isAssignmentExpression() &&
@@ -134,7 +134,7 @@ let memberExpressionOptimisationVisitor = {
       // optimise single spread args in calls
       // ex: fn(...args)
       if (state.offset === 0 && parentPath.isSpreadElement()) {
-        let call = parentPath.parentPath;
+        const call = parentPath.parentPath;
         if (call.isCallExpression() && call.node.arguments.length === 1) {
           state.candidates.push({cause: "argSpread", path});
           return;
@@ -175,7 +175,7 @@ function optimiseIndexGetter(path, argsId, offset) {
 
   const { scope } = path;
   if (!scope.isPure(index)) {
-    let temp = scope.generateUidIdentifierBasedOnNode(index);
+    const temp = scope.generateUidIdentifierBasedOnNode(index);
     scope.push({id: temp, kind: "var"});
     path.parentPath.replaceWith(restIndexImpure({
       ARGUMENTS: argsId,
@@ -201,20 +201,20 @@ function optimiseLengthGetter(path, argsId, offset) {
   }
 }
 
-export let visitor = {
+export const visitor = {
   Function(path) {
-    let { node, scope } = path;
+    const { node, scope } = path;
     if (!hasRest(node)) return;
 
-    let rest = node.params.pop().argument;
+    const rest = node.params.pop().argument;
 
-    let argsId = t.identifier("arguments");
+    const argsId = t.identifier("arguments");
 
     // otherwise `arguments` will be remapped in arrow functions
     argsId._shadowedFunctionLiteral = path;
 
     // check and optimise for extremely common cases
-    let state = {
+    const state = {
       references: [],
       offset:     node.params.length,
 
@@ -246,7 +246,7 @@ export let visitor = {
 
     // There are only "shorthand" references
     if (!state.deopted && !state.references.length) {
-      for (let {path, cause} of (state.candidates: Array)) {
+      for (const {path, cause} of (state.candidates: Array)) {
         switch (cause) {
           case "indexGetter":
             optimiseIndexGetter(path, argsId, state.offset);
@@ -268,9 +268,9 @@ export let visitor = {
     // deopt shadowed functions as transforms like regenerator may try touch the allocation loop
     state.deopted = state.deopted || !!node.shadow;
 
-    let start = t.numericLiteral(node.params.length);
-    let key = scope.generateUidIdentifier("key");
-    let len = scope.generateUidIdentifier("len");
+    const start = t.numericLiteral(node.params.length);
+    const key = scope.generateUidIdentifier("key");
+    const len = scope.generateUidIdentifier("len");
 
     let arrKey = key;
     let arrLen = len;
@@ -293,7 +293,7 @@ export let visitor = {
       );
     }
 
-    let loop = buildRest({
+    const loop = buildRest({
       ARGUMENTS: argsId,
       ARRAY_KEY: arrKey,
       ARRAY_LEN: arrLen,
diff --git a/packages/babel-plugin-transform-es2015-shorthand-properties/src/index.js b/packages/babel-plugin-transform-es2015-shorthand-properties/src/index.js
index c3aa80e4f5b9..3bc612ab0e5b 100644
--- a/packages/babel-plugin-transform-es2015-shorthand-properties/src/index.js
+++ b/packages/babel-plugin-transform-es2015-shorthand-properties/src/index.js
@@ -4,7 +4,7 @@ export default function () {
   return {
     visitor: {
       ObjectMethod(path) {
-        let { node } = path;
+        const { node } = path;
         if (node.kind === "method") {
           const func = t.functionExpression(null, node.params, node.body, node.generator, node.async);
           func.returnType = node.returnType;
diff --git a/packages/babel-plugin-transform-es2015-spread/src/index.js b/packages/babel-plugin-transform-es2015-spread/src/index.js
index d7768cb16129..14dfd5249249 100644
--- a/packages/babel-plugin-transform-es2015-spread/src/index.js
+++ b/packages/babel-plugin-transform-es2015-spread/src/index.js
@@ -17,7 +17,7 @@ export default function ({ types: t }) {
   }
 
   function build(props: Array, scope, state) {
-    let nodes = [];
+    const nodes = [];
 
     let _props = [];
 
@@ -27,7 +27,7 @@ export default function ({ types: t }) {
       _props = [];
     }
 
-    for (let prop of props) {
+    for (const prop of props) {
       if (t.isSpreadElement(prop)) {
         push();
         nodes.push(getSpreadLiteral(prop, scope, state));
@@ -44,11 +44,11 @@ export default function ({ types: t }) {
   return {
     visitor: {
       ArrayExpression(path, state) {
-        let { node, scope } = path;
-        let elements = node.elements;
+        const { node, scope } = path;
+        const elements = node.elements;
         if (!hasSpread(elements)) return;
 
-        let nodes = build(elements, scope, state);
+        const nodes = build(elements, scope, state);
         let first = nodes.shift();
 
         if (!t.isArrayExpression(first)) {
@@ -60,12 +60,12 @@ export default function ({ types: t }) {
       },
 
       CallExpression(path, state) {
-        let { node, scope } = path;
+        const { node, scope } = path;
 
-        let args = node.arguments;
+        const args = node.arguments;
         if (!hasSpread(args)) return;
 
-        let calleePath = path.get("callee");
+        const calleePath = path.get("callee");
         if (calleePath.isSuper()) return;
 
         let contextLiteral = t.identifier("undefined");
@@ -79,17 +79,17 @@ export default function ({ types: t }) {
           nodes = build(args, scope, state);
         }
 
-        let first = nodes.shift();
+        const first = nodes.shift();
         if (nodes.length) {
           node.arguments.push(t.callExpression(t.memberExpression(first, t.identifier("concat")), nodes));
         } else {
           node.arguments.push(first);
         }
 
-        let callee = node.callee;
+        const callee = node.callee;
 
         if (calleePath.isMemberExpression()) {
-          let temp = scope.maybeGenerateMemoised(callee.object);
+          const temp = scope.maybeGenerateMemoised(callee.object);
           if (temp) {
             callee.object = t.assignmentExpression("=", temp, callee.object);
             contextLiteral = temp;
@@ -109,13 +109,13 @@ export default function ({ types: t }) {
       },
 
       NewExpression(path, state) {
-        let { node, scope } = path;
+        const { node, scope } = path;
         let args = node.arguments;
         if (!hasSpread(args)) return;
 
-        let nodes = build(args, scope, state);
+        const nodes = build(args, scope, state);
 
-        let context = t.arrayExpression([t.nullLiteral()]);
+        const context = t.arrayExpression([t.nullLiteral()]);
 
 
         args = t.callExpression(t.memberExpression(context, t.identifier("concat")), nodes);
diff --git a/packages/babel-plugin-transform-es2015-sticky-regex/src/index.js b/packages/babel-plugin-transform-es2015-sticky-regex/src/index.js
index 8b703e85c810..409b6d9027d4 100644
--- a/packages/babel-plugin-transform-es2015-sticky-regex/src/index.js
+++ b/packages/babel-plugin-transform-es2015-sticky-regex/src/index.js
@@ -5,7 +5,7 @@ export default function () {
   return {
     visitor: {
       RegExpLiteral(path) {
-        let { node } = path;
+        const { node } = path;
         if (!regex.is(node, "y")) return;
 
         path.replaceWith(t.newExpression(t.identifier("RegExp"), [
diff --git a/packages/babel-plugin-transform-es2015-template-literals/src/index.js b/packages/babel-plugin-transform-es2015-template-literals/src/index.js
index 7359598f9982..40d3a4d24b31 100644
--- a/packages/babel-plugin-transform-es2015-template-literals/src/index.js
+++ b/packages/babel-plugin-transform-es2015-template-literals/src/index.js
@@ -10,14 +10,14 @@ export default function ({ types: t }) {
   return {
     visitor: {
       TaggedTemplateExpression(path, state) {
-        let { node } = path;
-        let quasi = node.quasi;
+        const { node } = path;
+        const quasi = node.quasi;
         let args  = [];
 
         let strings = [];
         let raw     = [];
 
-        for (let elem of (quasi.quasis: Array)) {
+        for (const elem of (quasi.quasis: Array)) {
           strings.push(t.stringLiteral(elem.value.cooked));
           raw.push(t.stringLiteral(elem.value.raw));
         }
@@ -28,7 +28,7 @@ export default function ({ types: t }) {
         let templateName = "taggedTemplateLiteral";
         if (state.opts.loose) templateName += "Loose";
 
-        let templateObject = state.file.addTemplateObject(templateName, strings, raw);
+        const templateObject = state.file.addTemplateObject(templateName, strings, raw);
         args.push(templateObject);
 
         args = args.concat(quasi.expressions);
@@ -39,12 +39,12 @@ export default function ({ types: t }) {
       TemplateLiteral(path, state) {
         let nodes: Array<Object> = [];
 
-        let expressions = path.get("expressions");
+        const expressions = path.get("expressions");
 
-        for (let elem of (path.node.quasis: Array)) {
+        for (const elem of (path.node.quasis: Array)) {
           nodes.push(t.stringLiteral(elem.value.cooked));
 
-          let expr = expressions.shift();
+          const expr = expressions.shift();
           if (expr) {
             if (state.opts.spec && !expr.isBaseType("string") && !expr.isBaseType("number"))  {
               nodes.push(t.callExpression(t.identifier("String"), [expr.node]));
@@ -66,7 +66,7 @@ export default function ({ types: t }) {
         if (nodes.length > 1) {
           let root = buildBinaryExpression(nodes.shift(), nodes.shift());
 
-          for (let node of nodes) {
+          for (const node of nodes) {
             root = buildBinaryExpression(root, node);
           }
 
diff --git a/packages/babel-plugin-transform-es2015-typeof-symbol/src/index.js b/packages/babel-plugin-transform-es2015-typeof-symbol/src/index.js
index b2291af302f9..4cea5acd1a5e 100644
--- a/packages/babel-plugin-transform-es2015-typeof-symbol/src/index.js
+++ b/packages/babel-plugin-transform-es2015-typeof-symbol/src/index.js
@@ -1,7 +1,7 @@
 /* eslint max-len: 0 */
 
 export default function ({ types: t }) {
-  let IGNORE = Symbol();
+  const IGNORE = Symbol();
 
   return {
     visitor: {
@@ -14,23 +14,23 @@ export default function ({ types: t }) {
       },
 
       UnaryExpression(path) {
-        let { node, parent } = path;
+        const { node, parent } = path;
         if (node[IGNORE]) return;
         if (path.find((path) => path.node && !!path.node._generated)) return;
 
         if (path.parentPath.isBinaryExpression() && t.EQUALITY_BINARY_OPERATORS.indexOf(parent.operator) >= 0) {
           // optimise `typeof foo === "string"` since we can determine that they'll never need to handle symbols
-          let opposite = path.getOpposite();
+          const opposite = path.getOpposite();
           if (opposite.isLiteral() && opposite.node.value !== "symbol" && opposite.node.value !== "object") {
             return;
           }
         }
 
         if (node.operator === "typeof") {
-          let call = t.callExpression(this.addHelper("typeof"), [node.argument]);
+          const call = t.callExpression(this.addHelper("typeof"), [node.argument]);
           if (path.get("argument").isIdentifier()) {
-            let undefLiteral = t.stringLiteral("undefined");
-            let unary = t.unaryExpression("typeof", node.argument);
+            const undefLiteral = t.stringLiteral("undefined");
+            const unary = t.unaryExpression("typeof", node.argument);
             unary[IGNORE] = true;
             path.replaceWith(t.conditionalExpression(
               t.binaryExpression("===", unary, undefLiteral),
diff --git a/packages/babel-plugin-transform-es3-member-expression-literals/src/index.js b/packages/babel-plugin-transform-es3-member-expression-literals/src/index.js
index fa78563a2348..2aa02f12083f 100644
--- a/packages/babel-plugin-transform-es3-member-expression-literals/src/index.js
+++ b/packages/babel-plugin-transform-es3-member-expression-literals/src/index.js
@@ -3,7 +3,7 @@ export default function ({ types: t }) {
     visitor: {
       MemberExpression: {
         exit({ node }) {
-          let prop = node.property;
+          const prop = node.property;
           if (!node.computed && t.isIdentifier(prop) && !t.isValidIdentifier(prop.name)) {
             // foo.default -> foo["default"]
             node.property = t.stringLiteral(prop.name);
diff --git a/packages/babel-plugin-transform-es3-property-literals/src/index.js b/packages/babel-plugin-transform-es3-property-literals/src/index.js
index b8f6f412c2b8..e62a88f8ee27 100644
--- a/packages/babel-plugin-transform-es3-property-literals/src/index.js
+++ b/packages/babel-plugin-transform-es3-property-literals/src/index.js
@@ -3,7 +3,7 @@ export default function ({ types: t }) {
     visitor: {
       ObjectProperty: {
         exit({node}) {
-          let key = node.key;
+          const key = node.key;
           if (!node.computed && t.isIdentifier(key) && !t.isValidIdentifier(key.name)) {
             // default: "bar" -> "default": "bar"
             node.key = t.stringLiteral(key.name);
diff --git a/packages/babel-plugin-transform-es5-property-mutators/src/index.js b/packages/babel-plugin-transform-es5-property-mutators/src/index.js
index 46d810f6f55a..83727bb0a59a 100644
--- a/packages/babel-plugin-transform-es5-property-mutators/src/index.js
+++ b/packages/babel-plugin-transform-es5-property-mutators/src/index.js
@@ -4,9 +4,9 @@ export default function ({ types: t }) {
   return {
     visitor: {
       ObjectExpression(path, file) {
-        let { node } = path;
+        const { node } = path;
         let hasAny = false;
-        for (let prop of (node.properties: Array)) {
+        for (const prop of (node.properties: Array)) {
           if (prop.kind === "get" || prop.kind === "set") {
             hasAny = true;
             break;
@@ -14,7 +14,7 @@ export default function ({ types: t }) {
         }
         if (!hasAny) return;
 
-        let mutatorMap = {};
+        const mutatorMap = {};
 
         node.properties = node.properties.filter(function (prop) {
           if (!prop.computed && (prop.kind === "get" || prop.kind === "set")) {
diff --git a/packages/babel-plugin-transform-eval/src/index.js b/packages/babel-plugin-transform-eval/src/index.js
index 677401057a40..ec92ec40f262 100644
--- a/packages/babel-plugin-transform-eval/src/index.js
+++ b/packages/babel-plugin-transform-eval/src/index.js
@@ -3,13 +3,13 @@ export default function ({ parse, traverse }) {
     visitor: {
       CallExpression(path) {
         if (path.get("callee").isIdentifier({ name: "eval" }) && path.node.arguments.length === 1) {
-          let evaluate = path.get("arguments")[0].evaluate();
+          const evaluate = path.get("arguments")[0].evaluate();
           if (!evaluate.confident) return;
 
-          let code = evaluate.value;
+          const code = evaluate.value;
           if (typeof code !== "string") return;
 
-          let ast = parse(code);
+          const ast = parse(code);
           traverse.removeProperties(ast);
           return ast.program;
         }
diff --git a/packages/babel-plugin-transform-export-extensions/src/index.js b/packages/babel-plugin-transform-export-extensions/src/index.js
index 6b0afad0ceb7..5951b9e42d91 100644
--- a/packages/babel-plugin-transform-export-extensions/src/index.js
+++ b/packages/babel-plugin-transform-export-extensions/src/index.js
@@ -1,10 +1,10 @@
 export default function ({ types: t }) {
   function build(node, nodes, scope) {
-    let first = node.specifiers[0];
+    const first = node.specifiers[0];
     if (!t.isExportNamespaceSpecifier(first) && !t.isExportDefaultSpecifier(first)) return;
 
-    let specifier = node.specifiers.shift();
-    let uid = scope.generateUidIdentifier(specifier.exported.name);
+    const specifier = node.specifiers.shift();
+    const uid = scope.generateUidIdentifier(specifier.exported.name);
 
     let newSpecifier;
     if (t.isExportNamespaceSpecifier(specifier)) {
@@ -24,8 +24,8 @@ export default function ({ types: t }) {
 
     visitor: {
       ExportNamedDeclaration(path) {
-        let { node, scope } = path;
-        let nodes = [];
+        const { node, scope } = path;
+        const nodes = [];
         build(node, nodes, scope);
         if (!nodes.length) return;
 
diff --git a/packages/babel-plugin-transform-flow-comments/src/index.js b/packages/babel-plugin-transform-flow-comments/src/index.js
index ece00a024a1c..8abf61aa41cf 100644
--- a/packages/babel-plugin-transform-flow-comments/src/index.js
+++ b/packages/babel-plugin-transform-flow-comments/src/index.js
@@ -16,14 +16,14 @@ export default function ({ types: t }) {
 
     visitor: {
       TypeCastExpression(path) {
-        let { node } = path;
+        const { node } = path;
         path.get("expression").addComment("trailing", generateComment(path.get("typeAnnotation")));
         path.replaceWith(t.parenthesizedExpression(node.expression));
       },
 
       // support function a(b?) {}
       Identifier(path) {
-        let { node } = path;
+        const { node } = path;
         if (!node.optional || node.typeAnnotation) {
           return;
         }
@@ -45,13 +45,13 @@ export default function ({ types: t }) {
 
       // support for `class X { foo: string }` - #4622
       ClassProperty(path) {
-        let { node, parent } = path;
+        const { node, parent } = path;
         if (!node.value) wrapInFlowComment(path, parent);
       },
 
       // support `export type a = {}` - #8 Error: You passed path.replaceWith() a falsy node
       "ExportNamedDeclaration|Flow"(path) {
-        let { node, parent } = path;
+        const { node, parent } = path;
         if (t.isExportNamedDeclaration(node) && !t.isFlow(node.declaration)) {
           return;
         }
@@ -60,7 +60,7 @@ export default function ({ types: t }) {
 
       // support `import type A` and `import typeof A` #10
       ImportDeclaration(path) {
-        let { node, parent } = path;
+        const { node, parent } = path;
         if (t.isImportDeclaration(node) && node.importKind !== "type" && node.importKind !== "typeof") {
           return;
         }
diff --git a/packages/babel-plugin-transform-flow-strip-types/src/index.js b/packages/babel-plugin-transform-flow-strip-types/src/index.js
index c64abb9a52fe..4b080f92580a 100644
--- a/packages/babel-plugin-transform-flow-strip-types/src/index.js
+++ b/packages/babel-plugin-transform-flow-strip-types/src/index.js
@@ -6,7 +6,7 @@ export default function ({ types: t }) {
 
     visitor: {
       Program(path, { file: { ast: { comments } } }) {
-        for (let comment of (comments: Array<Object>)) {
+        for (const comment of (comments: Array<Object>)) {
           if (comment.value.indexOf(FLOW_DIRECTIVE) >= 0) {
             // remove flow directive
             comment.value = comment.value.replace(FLOW_DIRECTIVE, "");
@@ -46,7 +46,7 @@ export default function ({ types: t }) {
 
       Function({ node }) {
         for (let i = 0; i < node.params.length; i++) {
-          let param = node.params[i];
+          const param = node.params[i];
           param.optional = false;
         }
       },
diff --git a/packages/babel-plugin-transform-function-bind/src/index.js b/packages/babel-plugin-transform-function-bind/src/index.js
index 0d3099930f53..27c20faa1e36 100644
--- a/packages/babel-plugin-transform-function-bind/src/index.js
+++ b/packages/babel-plugin-transform-function-bind/src/index.js
@@ -8,15 +8,15 @@ export default function ({ types: t }) {
   }
 
   function getStaticContext(bind, scope) {
-    let object = bind.object || bind.callee.object;
+    const object = bind.object || bind.callee.object;
     return scope.isStatic(object) && object;
   }
 
   function inferBindContext(bind, scope) {
-    let staticContext = getStaticContext(bind, scope);
+    const staticContext = getStaticContext(bind, scope);
     if (staticContext) return staticContext;
 
-    let tempId = getTempId(scope);
+    const tempId = getTempId(scope);
     if (bind.object) {
       bind.callee = t.sequenceExpression([
         t.assignmentExpression("=", tempId, bind.object),
@@ -33,17 +33,17 @@ export default function ({ types: t }) {
 
     visitor: {
       CallExpression({ node, scope }) {
-        let bind = node.callee;
+        const bind = node.callee;
         if (!t.isBindExpression(bind)) return;
 
-        let context = inferBindContext(bind, scope);
+        const context = inferBindContext(bind, scope);
         node.callee = t.memberExpression(bind.callee, t.identifier("call"));
         node.arguments.unshift(context);
       },
 
       BindExpression(path) {
-        let { node, scope } = path;
-        let context = inferBindContext(node, scope);
+        const { node, scope } = path;
+        const context = inferBindContext(node, scope);
         path.replaceWith(t.callExpression(t.memberExpression(node.callee, t.identifier("bind")), [context]));
       }
     }
diff --git a/packages/babel-plugin-transform-jscript/src/index.js b/packages/babel-plugin-transform-jscript/src/index.js
index 1361821acf05..87f83c2bedeb 100644
--- a/packages/babel-plugin-transform-jscript/src/index.js
+++ b/packages/babel-plugin-transform-jscript/src/index.js
@@ -3,7 +3,7 @@ export default function ({ types: t }) {
     visitor: {
       FunctionExpression: {
         exit(path) {
-          let { node } = path;
+          const { node } = path;
           if (!node.id) return;
           node._ignoreUserWhitespace = true;
 
diff --git a/packages/babel-plugin-transform-object-rest-spread/src/index.js b/packages/babel-plugin-transform-object-rest-spread/src/index.js
index dbc357be07d9..b705529236b4 100644
--- a/packages/babel-plugin-transform-object-rest-spread/src/index.js
+++ b/packages/babel-plugin-transform-object-rest-spread/src/index.js
@@ -11,7 +11,7 @@ export default function ({ types: t }) {
   }
 
   function hasSpread(node) {
-    for (let prop of (node.properties)) {
+    for (const prop of (node.properties)) {
       if (t.isSpreadProperty(prop)) {
         return true;
       }
@@ -22,8 +22,8 @@ export default function ({ types: t }) {
   function createObjectSpread(file, props, objRef) {
     const restProperty = props.pop();
 
-    let keys = [];
-    for (let prop of props) {
+    const keys = [];
+    for (const prop of props) {
       let key = prop.key;
       if (t.isIdentifier(key) && !prop.computed) {
         key = t.stringLiteral(prop.key.name);
@@ -44,10 +44,10 @@ export default function ({ types: t }) {
 
   function replaceRestProperty(paramsPath, i, numParams) {
     if (paramsPath.isObjectPattern() && hasRestProperty(paramsPath)) {
-      let parentPath = paramsPath.parentPath;
-      let uid = parentPath.scope.generateUidIdentifier("ref");
+      const parentPath = paramsPath.parentPath;
+      const uid = parentPath.scope.generateUidIdentifier("ref");
 
-      let declar = t.variableDeclaration("let", [
+      const declar = t.variableDeclaration("let", [
         t.variableDeclarator(paramsPath.node, uid)
       ]);
       declar._blockHoist = i ? numParams - i : 1;
@@ -65,7 +65,7 @@ export default function ({ types: t }) {
       // taken from transform-es2015-parameters/src/destructuring.js
       // function a({ b, ...c }) {}
       Function(path) {
-        let params = path.get("params");
+        const params = path.get("params");
         for (let i = 0; i < params.length; i++) {
           replaceRestProperty(params[i], i, params.length);
         }
@@ -75,7 +75,7 @@ export default function ({ types: t }) {
       VariableDeclarator(path, file) {
         if (!path.get("id").isObjectPattern()) { return; }
         const kind = path.parentPath.node.kind;
-        let nodes = [];
+        const nodes = [];
 
         path.traverse({
           RestProperty(path) {
@@ -89,7 +89,7 @@ export default function ({ types: t }) {
               }
             });
 
-            let [ argument, callExpression ] = createObjectSpread(
+            const [ argument, callExpression ] = createObjectSpread(
               file,
               path.parentPath.node.properties,
               ref
@@ -108,7 +108,7 @@ export default function ({ types: t }) {
               ).remove();
             }
           }
-        },{
+        }, {
           originalPath: path
         });
 
@@ -122,14 +122,14 @@ export default function ({ types: t }) {
       // taken from transform-es2015-destructuring/src/index.js#visitor
       // export var { a, ...b } = c;
       ExportNamedDeclaration(path) {
-        let declaration = path.get("declaration");
+        const declaration = path.get("declaration");
         if (!declaration.isVariableDeclaration()) return;
         if (!hasRestProperty(declaration)) return;
 
-        let specifiers = [];
+        const specifiers = [];
 
-        for (let name in path.getOuterBindingIdentifiers(path)) {
-          let id = t.identifier(name);
+        for (const name in path.getOuterBindingIdentifiers(path)) {
+          const id = t.identifier(name);
           specifiers.push(t.exportSpecifier(id, id));
         }
 
@@ -145,9 +145,9 @@ export default function ({ types: t }) {
       },
       // ({a, ...b} = c);
       AssignmentExpression(path, file) {
-        let leftPath = path.get("left");
+        const leftPath = path.get("left");
         if (leftPath.isObjectPattern() && hasRestProperty(leftPath)) {
-          let nodes = [];
+          const nodes = [];
 
           let ref;
           if (path.isCompletionRecord() || path.parentPath.isExpressionStatement()) {
@@ -158,13 +158,13 @@ export default function ({ types: t }) {
             ]));
           }
 
-          let [ argument, callExpression ] = createObjectSpread(
+          const [ argument, callExpression ] = createObjectSpread(
             file,
             path.node.left.properties,
             ref
           );
 
-          let nodeWithoutSpread = t.clone(path.node);
+          const nodeWithoutSpread = t.clone(path.node);
           nodeWithoutSpread.right = ref;
           nodes.push(t.expressionStatement(nodeWithoutSpread));
           nodes.push(t.toStatement(t.assignmentExpression(
@@ -182,13 +182,13 @@ export default function ({ types: t }) {
       },
       // taken from transform-es2015-destructuring/src/index.js#visitor
       ForXStatement(path) {
-        let { node, scope } = path;
-        let leftPath = path.get("left");
-        let left = node.left;
+        const { node, scope } = path;
+        const leftPath = path.get("left");
+        const left = node.left;
 
         // for ({a, ...b} of []) {}
         if (t.isObjectPattern(left) && hasRestProperty(leftPath)) {
-          let temp = scope.generateUidIdentifier("ref");
+          const temp = scope.generateUidIdentifier("ref");
 
           node.left = t.variableDeclaration("var", [
             t.variableDeclarator(temp)
@@ -205,10 +205,10 @@ export default function ({ types: t }) {
 
         if (!t.isVariableDeclaration(left)) return;
 
-        let pattern = left.declarations[0].id;
+        const pattern = left.declarations[0].id;
         if (!t.isObjectPattern(pattern)) return;
 
-        let key = scope.generateUidIdentifier("ref");
+        const key = scope.generateUidIdentifier("ref");
         node.left = t.variableDeclaration(left.kind, [
           t.variableDeclarator(key, null)
         ]);
@@ -225,12 +225,12 @@ export default function ({ types: t }) {
       ObjectExpression(path, file) {
         if (!hasSpread(path.node)) return;
 
-        let useBuiltIns = file.opts.useBuiltIns || false;
+        const useBuiltIns = file.opts.useBuiltIns || false;
         if (typeof useBuiltIns !== "boolean") {
           throw new Error("transform-object-rest-spread currently only accepts a boolean option for useBuiltIns (defaults to false)");
         }
 
-        let args = [];
+        const args = [];
         let props = [];
 
         function push() {
@@ -239,7 +239,7 @@ export default function ({ types: t }) {
           props = [];
         }
 
-        for (let prop of (path.node.properties: Array)) {
+        for (const prop of (path.node.properties: Array)) {
           if (t.isSpreadProperty(prop)) {
             push();
             args.push(prop.argument);
diff --git a/packages/babel-plugin-transform-proto-to-assign/src/index.js b/packages/babel-plugin-transform-proto-to-assign/src/index.js
index d17f857c187d..41b0ff442d4a 100644
--- a/packages/babel-plugin-transform-proto-to-assign/src/index.js
+++ b/packages/babel-plugin-transform-proto-to-assign/src/index.js
@@ -8,7 +8,7 @@ export default function ({ types: t }) {
   }
 
   function isProtoAssignmentExpression(node) {
-    let left = node.left;
+    const left = node.left;
     return t.isMemberExpression(left) && t.isLiteral(t.toComputedKey(left, left.property), { value: "__proto__" });
   }
 
@@ -21,9 +21,9 @@ export default function ({ types: t }) {
       AssignmentExpression(path, file) {
         if (!isProtoAssignmentExpression(path.node)) return;
 
-        let nodes = [];
-        let left  = path.node.left.object;
-        let temp  = path.scope.maybeGenerateMemoised(left);
+        const nodes = [];
+        const left  = path.node.left.object;
+        const temp  = path.scope.maybeGenerateMemoised(left);
 
         if (temp) nodes.push(t.expressionStatement(t.assignmentExpression("=", temp, left)));
         nodes.push(buildDefaultsCallExpression(path.node, temp || left, file));
@@ -33,7 +33,7 @@ export default function ({ types: t }) {
       },
 
       ExpressionStatement(path, file) {
-        let expr = path.node.expression;
+        const expr = path.node.expression;
         if (!t.isAssignmentExpression(expr, { operator: "=" })) return;
 
         if (isProtoAssignmentExpression(expr)) {
@@ -43,9 +43,9 @@ export default function ({ types: t }) {
 
       ObjectExpression(path, file) {
         let proto;
-        let { node } = path;
+        const { node } = path;
 
-        for (let prop of (node.properties: Array)) {
+        for (const prop of (node.properties: Array)) {
           if (isProtoKey(prop)) {
             proto = prop.value;
             pull(node.properties, prop);
@@ -53,7 +53,7 @@ export default function ({ types: t }) {
         }
 
         if (proto) {
-          let args = [t.objectExpression([]), proto];
+          const args = [t.objectExpression([]), proto];
           if (node.properties.length) args.push(node);
           path.replaceWith(t.callExpression(file.addHelper("extends"), args));
         }
diff --git a/packages/babel-plugin-transform-react-constant-elements/src/index.js b/packages/babel-plugin-transform-react-constant-elements/src/index.js
index 9673af7c2683..a25fb67e6c6d 100644
--- a/packages/babel-plugin-transform-react-constant-elements/src/index.js
+++ b/packages/babel-plugin-transform-react-constant-elements/src/index.js
@@ -1,7 +1,7 @@
 export default function () {
-  let immutabilityVisitor = {
+  const immutabilityVisitor = {
     enter(path, state) {
-      let stop = () => {
+      const stop = () => {
         state.isImmutable = false;
         path.stop();
       };
@@ -28,7 +28,7 @@ export default function () {
       JSXElement(path) {
         if (path.node._hoisted) return;
 
-        let state = { isImmutable: true };
+        const state = { isImmutable: true };
         path.traverse(immutabilityVisitor, state);
 
         if (state.isImmutable) {
diff --git a/packages/babel-plugin-transform-react-display-name/src/index.js b/packages/babel-plugin-transform-react-display-name/src/index.js
index 43206810e7e9..ce73d4db74a3 100644
--- a/packages/babel-plugin-transform-react-display-name/src/index.js
+++ b/packages/babel-plugin-transform-react-display-name/src/index.js
@@ -2,12 +2,12 @@ import path from "path";
 
 export default function ({ types: t }) {
   function addDisplayName(id, call) {
-    let props = call.arguments[0].properties;
+    const props = call.arguments[0].properties;
     let safe = true;
 
     for (let i = 0; i < props.length; i++) {
-      let prop = props[i];
-      let key = t.toComputedKey(prop);
+      const prop = props[i];
+      const key = t.toComputedKey(prop);
       if (t.isLiteral(key, { value: "displayName" })) {
         safe = false;
         break;
@@ -19,7 +19,7 @@ export default function ({ types: t }) {
     }
   }
 
-  let isCreateClassCallExpression = t.buildMatchMemberExpression("React.createClass");
+  const isCreateClassCallExpression = t.buildMatchMemberExpression("React.createClass");
 
   function isCreateClass(node) {
     if (!node || !t.isCallExpression(node)) return false;
@@ -28,11 +28,11 @@ export default function ({ types: t }) {
     if (!isCreateClassCallExpression(node.callee)) return false;
 
     // no call arguments
-    let args = node.arguments;
+    const args = node.arguments;
     if (args.length !== 1) return false;
 
     // first node arg is not an object
-    let first = args[0];
+    const first = args[0];
     if (!t.isObjectExpression(first)) return false;
 
     return true;
@@ -54,7 +54,7 @@ export default function ({ types: t }) {
       },
 
       CallExpression(path) {
-        let { node } = path;
+        const { node } = path;
         if (!isCreateClass(node)) return;
 
         let id;
diff --git a/packages/babel-plugin-transform-react-inline-elements/src/index.js b/packages/babel-plugin-transform-react-inline-elements/src/index.js
index 5f394fc3f8b8..cb0e5a12c11e 100644
--- a/packages/babel-plugin-transform-react-inline-elements/src/index.js
+++ b/packages/babel-plugin-transform-react-inline-elements/src/index.js
@@ -1,7 +1,7 @@
 export default function ({ types: t }) {
   function hasRefOrSpread(attrs) {
     for (let i = 0; i < attrs.length; i++) {
-      let attr = attrs[i];
+      const attr = attrs[i];
       if (t.isJSXSpreadAttribute(attr)) return true;
       if (isJSXAttributeOfName(attr, "ref")) return true;
     }
@@ -22,14 +22,14 @@ export default function ({ types: t }) {
   return {
     visitor: {
       JSXElement(path, file) {
-        let { node } = path;
+        const { node } = path;
 
         // filter
-        let open = node.openingElement;
+        const open = node.openingElement;
         if (hasRefOrSpread(open.attributes)) return;
 
         // init
-        let props       = t.objectExpression([]);
+        const props       = t.objectExpression([]);
         let key         = null;
         let type        = open.name;
 
@@ -42,26 +42,26 @@ export default function ({ types: t }) {
         }
 
         // props
-        for (let attr of (open.attributes: Array<Object>)) {
+        for (const attr of (open.attributes: Array<Object>)) {
           if (isJSXAttributeOfName(attr, "key")) {
             key = getAttributeValue(attr);
           } else {
-            let name = attr.name.name;
-            let propertyKey = t.isValidIdentifier(name) ? t.identifier(name) : t.stringLiteral(name);
+            const name = attr.name.name;
+            const propertyKey = t.isValidIdentifier(name) ? t.identifier(name) : t.stringLiteral(name);
             pushProp(props.properties, propertyKey, getAttributeValue(attr));
           }
         }
 
-        let args = [type, props];
+        const args = [type, props];
         if (key || node.children.length) {
-          let children = t.react.buildChildren(node);
+          const children = t.react.buildChildren(node);
           args.push(
             key || t.unaryExpression("void", t.numericLiteral(0), true),
             ...children
           );
         }
 
-        let el = t.callExpression(file.addHelper("jsx"), args);
+        const el = t.callExpression(file.addHelper("jsx"), args);
         path.replaceWith(el);
       }
     }
diff --git a/packages/babel-plugin-transform-react-jsx-self/src/index.js b/packages/babel-plugin-transform-react-jsx-self/src/index.js
index 2fd02a77ed05..39f223df6435 100644
--- a/packages/babel-plugin-transform-react-jsx-self/src/index.js
+++ b/packages/babel-plugin-transform-react-jsx-self/src/index.js
@@ -15,7 +15,7 @@
 const TRACE_ID = "__self";
 
 export default function ({ types: t }) {
-  let visitor = {
+  const visitor = {
     JSXOpeningElement({ node }) {
       const id = t.jSXIdentifier(TRACE_ID);
       const trace = t.thisExpression();
diff --git a/packages/babel-plugin-transform-react-jsx-source/src/index.js b/packages/babel-plugin-transform-react-jsx-source/src/index.js
index cd296af979a0..039e9e5adc60 100644
--- a/packages/babel-plugin-transform-react-jsx-source/src/index.js
+++ b/packages/babel-plugin-transform-react-jsx-source/src/index.js
@@ -26,7 +26,7 @@ export default function ({ types: t }) {
     return t.objectExpression([fileNameProperty, lineNumberProperty]);
   }
 
-  let visitor = {
+  const visitor = {
     JSXOpeningElement(path, state) {
       const id = t.jSXIdentifier(TRACE_ID);
       const location = path.container.openingElement.loc;
diff --git a/packages/babel-plugin-transform-react-jsx/src/index.js b/packages/babel-plugin-transform-react-jsx/src/index.js
index 0a80ffac4bfa..12e127c90929 100644
--- a/packages/babel-plugin-transform-react-jsx/src/index.js
+++ b/packages/babel-plugin-transform-react-jsx/src/index.js
@@ -1,12 +1,12 @@
 /* eslint max-len: 0 */
 
 export default function ({ types: t }) {
-  let JSX_ANNOTATION_REGEX = /\*?\s*@jsx\s+([^\s]+)/;
+  const JSX_ANNOTATION_REGEX = /\*?\s*@jsx\s+([^\s]+)/;
 
-  let visitor = require("babel-helper-builder-react-jsx")({
+  const visitor = require("babel-helper-builder-react-jsx")({
     pre(state) {
-      let tagName = state.tagName;
-      let args    = state.args;
+      const tagName = state.tagName;
+      const args    = state.args;
       if (t.react.isCompatTag(tagName)) {
         args.push(t.stringLiteral(tagName));
       } else {
@@ -20,11 +20,11 @@ export default function ({ types: t }) {
   });
 
   visitor.Program = function (path, state) {
-    let { file } = state;
+    const { file } = state;
     let id = state.opts.pragma || "React.createElement";
 
-    for (let comment of (file.ast.comments: Array<Object>)) {
-      let matches = JSX_ANNOTATION_REGEX.exec(comment.value);
+    for (const comment of (file.ast.comments: Array<Object>)) {
+      const matches = JSX_ANNOTATION_REGEX.exec(comment.value);
       if (matches) {
         id = matches[1];
         if (id === "React.DOM") {
diff --git a/packages/babel-plugin-transform-runtime/src/index.js b/packages/babel-plugin-transform-runtime/src/index.js
index 805df686ae10..8299547e911e 100644
--- a/packages/babel-plugin-transform-runtime/src/index.js
+++ b/packages/babel-plugin-transform-runtime/src/index.js
@@ -9,7 +9,7 @@ export default function ({ types: t }) {
     return Object.prototype.hasOwnProperty.call(obj, key);
   }
 
-  let HELPER_BLACKLIST = ["interopRequireWildcard", "interopRequireDefault"];
+  const HELPER_BLACKLIST = ["interopRequireWildcard", "interopRequireDefault"];
 
   return {
     pre(file) {
@@ -30,7 +30,7 @@ export default function ({ types: t }) {
 
     visitor: {
       ReferencedIdentifier(path, state) {
-        let { node, parent, scope } = path;
+        const { node, parent, scope } = path;
 
         if (node.name === "regeneratorRuntime" && state.opts.regenerator !== false) {
           path.replaceWith(state.get("regeneratorIdentifier"));
@@ -59,7 +59,7 @@ export default function ({ types: t }) {
         // we can't compile this
         if (path.node.arguments.length) return;
 
-        let callee = path.node.callee;
+        const callee = path.node.callee;
         if (!t.isMemberExpression(callee)) return;
         if (!callee.computed) return;
         if (!path.get("callee.property").matchesPattern("Symbol.iterator")) return;
@@ -99,15 +99,15 @@ export default function ({ types: t }) {
           if (state.opts.polyfill === false) return;
           if (!path.isReferenced()) return;
 
-          let { node } = path;
-          let obj = node.object;
-          let prop = node.property;
+          const { node } = path;
+          const obj = node.object;
+          const prop = node.property;
 
           if (!t.isReferenced(obj, node)) return;
           if (node.computed) return;
           if (!has(definitions.methods, obj.name)) return;
 
-          let methods = definitions.methods[obj.name];
+          const methods = definitions.methods[obj.name];
           if (!has(methods, prop.name)) return;
 
           // doesn't reference the global
@@ -115,7 +115,7 @@ export default function ({ types: t }) {
 
           // special case Object.defineProperty to not use core-js when using string keys
           if (obj.name === "Object" && prop.name === "defineProperty" && path.parentPath.isCallExpression()) {
-            let call = path.parentPath.node;
+            const call = path.parentPath.node;
             if (call.arguments.length === 3 && t.isLiteral(call.arguments[1])) return;
           }
 
@@ -131,8 +131,8 @@ export default function ({ types: t }) {
           if (state.opts.polyfill === false) return;
           if (!path.isReferenced()) return;
 
-          let { node } = path;
-          let obj = node.object;
+          const { node } = path;
+          const obj = node.object;
 
           if (!has(definitions.builtins, obj.name)) return;
           if (path.scope.getBindingIdentifier(obj.name)) return;
diff --git a/packages/babel-plugin-transform-strict-mode/src/index.js b/packages/babel-plugin-transform-strict-mode/src/index.js
index 8ed2d181fb4f..fbd1dd913710 100644
--- a/packages/babel-plugin-transform-strict-mode/src/index.js
+++ b/packages/babel-plugin-transform-strict-mode/src/index.js
@@ -6,9 +6,9 @@ export default function () {
       Program(path, state) {
         if (state.opts.strict === false || state.opts.strictMode === false) return;
 
-        let { node } = path;
+        const { node } = path;
 
-        for (let directive of (node.directives: Array<Object>)) {
+        for (const directive of (node.directives: Array<Object>)) {
           if (directive.value.value === "use strict") return;
         }
 
diff --git a/packages/babel-plugin-undeclared-variables-check/src/index.js b/packages/babel-plugin-undeclared-variables-check/src/index.js
index a6400dd8c7af..9c63b00aaba6 100644
--- a/packages/babel-plugin-undeclared-variables-check/src/index.js
+++ b/packages/babel-plugin-undeclared-variables-check/src/index.js
@@ -4,9 +4,9 @@ export default function ({ messages }) {
   return {
     visitor: {
       ReferencedIdentifier(path) {
-        let { node, scope } = path;
+        const { node, scope } = path;
 
-        let binding = scope.getBinding(node.name);
+        const binding = scope.getBinding(node.name);
         if (binding && binding.kind === "type" && !path.parentPath.isFlow()) {
           throw path.buildCodeFrameError(messages.get("undeclaredVariableType", node.name), ReferenceError);
         }
@@ -16,13 +16,13 @@ export default function ({ messages }) {
         // get the closest declaration to offer as a suggestion
         // the variable name may have just been mistyped
 
-        let bindings = scope.getAllBindings();
+        const bindings = scope.getAllBindings();
 
         let closest;
         let shortest = -1;
 
-        for (let name in bindings) {
-          let distance = leven(node.name, name);
+        for (const name in bindings) {
+          const distance = leven(node.name, name);
           if (distance <= 0 || distance > 3) continue;
           if (distance <= shortest) continue;
 
diff --git a/packages/babel-polyfill/src/index.js b/packages/babel-polyfill/src/index.js
index 0cb9b282d619..2dfde7b6d754 100644
--- a/packages/babel-polyfill/src/index.js
+++ b/packages/babel-polyfill/src/index.js
@@ -12,7 +12,7 @@ import "regenerator-runtime/runtime";
 
 import "core-js/fn/regexp/escape";
 
-let DEFINE_PROPERTY = "defineProperty";
+const DEFINE_PROPERTY = "defineProperty";
 function define(O, key, value) {
   O[key] || Object[DEFINE_PROPERTY](O, key, {
     writable:     true,
diff --git a/packages/babel-preset-es2015/test/index.js b/packages/babel-preset-es2015/test/index.js
index 7653dbb6f97e..8047319eb6b3 100644
--- a/packages/babel-preset-es2015/test/index.js
+++ b/packages/babel-preset-es2015/test/index.js
@@ -1,5 +1,5 @@
-let es2015 = require("../lib");
-let expect = require("chai").expect;
+const es2015 = require("../lib");
+const expect = require("chai").expect;
 
 describe("es2015 preset", function () {
   it("exposes an object", function () {
diff --git a/packages/babel-preset-es2015/test/traceur.js b/packages/babel-preset-es2015/test/traceur.js
index a4bfd87c3f1a..da8c33beaeef 100644
--- a/packages/babel-preset-es2015/test/traceur.js
+++ b/packages/babel-preset-es2015/test/traceur.js
@@ -1,4 +1,4 @@
-let _  = require("lodash");
+const _  = require("lodash");
 
 require("babel-helper-transform-fixture-test-runner")(__dirname + "/fixtures/traceur", "traceur", {
   ignoreSuites: [
diff --git a/packages/babel-register/src/node.js b/packages/babel-register/src/node.js
index 91edc8c65be8..19e052f5007a 100644
--- a/packages/babel-register/src/node.js
+++ b/packages/babel-register/src/node.js
@@ -12,7 +12,7 @@ sourceMapSupport.install({
   handleUncaughtExceptions: false,
   environment : "node",
   retrieveSourceMap(source) {
-    let map = maps && maps[source];
+    const map = maps && maps[source];
     if (map) {
       return {
         url: null,
@@ -27,15 +27,15 @@ sourceMapSupport.install({
 registerCache.load();
 let cache = registerCache.get();
 
-let transformOpts = {};
+const transformOpts = {};
 
 let ignore;
 let only;
 
 let oldHandlers   = {};
-let maps          = {};
+const maps          = {};
 
-let cwd = process.cwd();
+const cwd = process.cwd();
 
 function getRelativePath(filename) {
   return path.relative(cwd, filename);
@@ -49,7 +49,7 @@ function compile(filename) {
   let result;
 
   // merge in base options and resolve all the plugins and presets relative to this file
-  let opts = new OptionManager().init(extend(
+  const opts = new OptionManager().init(extend(
     { sourceRoot: path.dirname(filename) }, // sourceRoot can be overwritten
     deepClone(transformOpts),
     { filename }
@@ -57,11 +57,11 @@ function compile(filename) {
 
   let cacheKey = `${JSON.stringify(opts)}:${babel.version}`;
 
-  let env = process.env.BABEL_ENV || process.env.NODE_ENV;
+  const env = process.env.BABEL_ENV || process.env.NODE_ENV;
   if (env) cacheKey += `:${env}`;
 
   if (cache) {
-    let cached = cache[cacheKey];
+    const cached = cache[cacheKey];
     if (cached && cached.mtime === mtime(filename)) {
       result = cached;
     }
@@ -100,7 +100,7 @@ function loader(m, filename) {
 }
 
 function registerExtension(ext) {
-  let old = oldHandlers[ext] || oldHandlers[".js"] || require.extensions[".js"];
+  const old = oldHandlers[ext] || oldHandlers[".js"] || require.extensions[".js"];
 
   require.extensions[ext] = function (m, filename) {
     if (shouldIgnore(filename)) {
diff --git a/packages/babel-template/src/index.js b/packages/babel-template/src/index.js
index 5413e5d6ab90..4abd2a460803 100644
--- a/packages/babel-template/src/index.js
+++ b/packages/babel-template/src/index.js
@@ -7,8 +7,8 @@ import traverse from "babel-traverse";
 import * as babylon from "babylon";
 import * as t from "babel-types";
 
-let FROM_TEMPLATE = "_fromTemplate"; //Symbol(); // todo: probably wont get copied over
-let TEMPLATE_SKIP = Symbol();
+const FROM_TEMPLATE = "_fromTemplate"; //Symbol(); // todo: probably wont get copied over
+const TEMPLATE_SKIP = Symbol();
 
 export default function (code: string, opts?: Object): Function {
   // since we lazy parse the template, we get the current stack so we have the
@@ -60,7 +60,7 @@ export default function (code: string, opts?: Object): Function {
 
 function useTemplate(ast, nodes?: Array<Object>) {
   ast = cloneDeep(ast);
-  let { program } = ast;
+  const { program } = ast;
 
   if (nodes.length) {
     traverse(ast, templateVisitor, null, nodes);
@@ -73,7 +73,7 @@ function useTemplate(ast, nodes?: Array<Object>) {
   }
 }
 
-let templateVisitor = {
+const templateVisitor = {
   // 360
   noScope: true,
 
@@ -91,7 +91,7 @@ let templateVisitor = {
       if (has(args[0], node.name)) {
         replacement = args[0][node.name];
       } else if (node.name[0] === "$") {
-        let i = +node.name.slice(1);
+        const i = +node.name.slice(1);
         if (args[i]) replacement = args[i];
       }
     }
diff --git a/packages/babel-template/test/index.js b/packages/babel-template/test/index.js
index f64090e4a839..6e4ae78544c0 100644
--- a/packages/babel-template/test/index.js
+++ b/packages/babel-template/test/index.js
@@ -1,8 +1,8 @@
-let generator = require("../../babel-generator").default;
-let template  = require("../lib");
-let chai      = require("chai");
+const generator = require("../../babel-generator").default;
+const template  = require("../lib");
+const chai      = require("chai");
 
-let comments = "// Sum two numbers\nconst add = (a, b) => a + b;";
+const comments = "// Sum two numbers\nconst add = (a, b) => a + b;";
 
 describe("templating", function () {
   it("import statement will cause parser to throw by default", function () {
@@ -18,13 +18,13 @@ describe("templating", function () {
   });
 
   it("should strip comments by default", function () {
-    let code = "const add = (a, b) => a + b;";
-    let output = template(comments)();
+    const code = "const add = (a, b) => a + b;";
+    const output = template(comments)();
     chai.expect(generator(output).code).to.be.equal(code);
   });
 
   it("should preserve comments with a flag", function () {
-    let output = template(comments, {preserveComments: true})();
+    const output = template(comments, {preserveComments: true})();
     chai.expect(generator(output).code).to.be.equal(comments);
   });
 });
diff --git a/packages/babel-traverse/src/context.js b/packages/babel-traverse/src/context.js
index b73ff3f7bfb1..3be2d90f85d5 100644
--- a/packages/babel-traverse/src/context.js
+++ b/packages/babel-traverse/src/context.js
@@ -1,7 +1,7 @@
 import NodePath from "./path";
 import * as t from "babel-types";
 
-let testing = process.env.NODE_ENV === "test";
+const testing = process.env.NODE_ENV === "test";
 
 export default class TraversalContext {
   constructor(scope, opts, state, parentPath) {
@@ -23,18 +23,18 @@ export default class TraversalContext {
    */
 
   shouldVisit(node): boolean {
-    let opts = this.opts;
+    const opts = this.opts;
     if (opts.enter || opts.exit) return true;
 
     // check if we have a visitor for this node
     if (opts[node.type]) return true;
 
     // check if we're going to traverse into this node
-    let keys: ?Array<string> = t.VISITOR_KEYS[node.type];
+    const keys: ?Array<string> = t.VISITOR_KEYS[node.type];
     if (!keys || !keys.length) return false;
 
     // we need to traverse into this node so ensure that it has children to traverse into!
-    for (let key of keys) {
+    for (const key of keys) {
       if (node[key]) return true;
     }
 
@@ -69,11 +69,11 @@ export default class TraversalContext {
     // nothing to traverse!
     if (container.length === 0) return false;
 
-    let queue = [];
+    const queue = [];
 
     // build up initial queue
     for (let key = 0; key < container.length; key++) {
-      let node = container[key];
+      const node = container[key];
       if (node && this.shouldVisit(node)) {
         queue.push(this.create(parent, container, key, listKey));
       }
@@ -97,11 +97,11 @@ export default class TraversalContext {
     this.queue = queue;
     this.priorityQueue = [];
 
-    let visited = [];
+    const visited = [];
     let stop = false;
 
     // visit the queue
-    for (let path of queue) {
+    for (const path of queue) {
       path.resync();
 
       if (path.contexts.length === 0 || path.contexts[path.contexts.length - 1] !== this) {
@@ -136,7 +136,7 @@ export default class TraversalContext {
     }
 
     // clear queue
-    for (let path of queue) {
+    for (const path of queue) {
       path.popContext();
     }
 
@@ -147,7 +147,7 @@ export default class TraversalContext {
   }
 
   visit(node, key) {
-    let nodes = node[key];
+    const nodes = node[key];
     if (!nodes) return false;
 
     if (Array.isArray(nodes)) {
diff --git a/packages/babel-traverse/src/index.js b/packages/babel-traverse/src/index.js
index 6a3d4660a4b7..464a5a150fab 100644
--- a/packages/babel-traverse/src/index.js
+++ b/packages/babel-traverse/src/index.js
@@ -46,11 +46,11 @@ traverse.cheap = function (node, enter) {
 };
 
 traverse.node = function (node: Object, opts: Object, scope: Object, state: Object, parentPath: Object, skipKeys?) {
-  let keys: Array = t.VISITOR_KEYS[node.type];
+  const keys: Array = t.VISITOR_KEYS[node.type];
   if (!keys) return;
 
-  let context = new TraversalContext(scope, opts, state, parentPath);
-  for (let key of keys) {
+  const context = new TraversalContext(scope, opts, state, parentPath);
+  for (const key of keys) {
     if (skipKeys && skipKeys[key]) continue;
     if (context.visit(node, key)) return;
   }
@@ -81,7 +81,7 @@ traverse.hasType = function (tree: Object, scope: Object, type: Object, blacklis
   // the type we're looking for is the same as the passed node
   if (tree.type === type) return true;
 
-  let state = {
+  const state = {
     has:  false,
     type: type
   };
diff --git a/packages/babel-traverse/src/path/ancestry.js b/packages/babel-traverse/src/path/ancestry.js
index 35fb7b34359c..245910aa6a98 100644
--- a/packages/babel-traverse/src/path/ancestry.js
+++ b/packages/babel-traverse/src/path/ancestry.js
@@ -60,10 +60,10 @@ export function getStatementParent() {
 export function getEarliestCommonAncestorFrom(paths: Array<NodePath>): NodePath {
   return this.getDeepestCommonAncestorFrom(paths, function (deepest, i, ancestries) {
     let earliest;
-    let keys = t.VISITOR_KEYS[deepest.type];
+    const keys = t.VISITOR_KEYS[deepest.type];
 
-    for (let ancestry of (ancestries: Array)) {
-      let path = ancestry[i + 1];
+    for (const ancestry of (ancestries: Array)) {
+      const path = ancestry[i + 1];
 
       // first path
       if (!earliest) {
@@ -81,8 +81,8 @@ export function getEarliestCommonAncestorFrom(paths: Array<NodePath>): NodePath
       }
 
       // handle keys
-      let earliestKeyIndex = keys.indexOf(earliest.parentKey);
-      let currentKeyIndex = keys.indexOf(path.parentKey);
+      const earliestKeyIndex = keys.indexOf(earliest.parentKey);
+      const currentKeyIndex = keys.indexOf(path.parentKey);
       if (earliestKeyIndex > currentKeyIndex) {
         // key appears before so it's earlier
         earliest = path;
@@ -115,8 +115,8 @@ export function getDeepestCommonAncestorFrom(paths: Array<NodePath>, filter?: Fu
   let lastCommonIndex, lastCommon;
 
   // get the ancestors of the path, breaking when the parent exceeds ourselves
-  let ancestries = paths.map((path) => {
-    let ancestry = [];
+  const ancestries = paths.map((path) => {
+    const ancestry = [];
 
     do {
       ancestry.unshift(path);
@@ -131,13 +131,13 @@ export function getDeepestCommonAncestorFrom(paths: Array<NodePath>, filter?: Fu
   });
 
   // get the first ancestry so we have a seed to assess all other ancestries with
-  let first = ancestries[0];
+  const first = ancestries[0];
 
   // check ancestor equality
   depthLoop: for (let i = 0; i < minDepth; i++) {
-    let shouldMatch = first[i];
+    const shouldMatch = first[i];
 
-    for (let ancestry of (ancestries: Array)) {
+    for (const ancestry of (ancestries: Array)) {
       if (ancestry[i] !== shouldMatch) {
         // we've hit a snag
         break depthLoop;
@@ -168,7 +168,7 @@ export function getDeepestCommonAncestorFrom(paths: Array<NodePath>, filter?: Fu
 
 export function getAncestry() {
   let path = this;
-  let paths = [];
+  const paths = [];
   do {
     paths.push(path);
   } while (path = path.parentPath);
@@ -192,7 +192,7 @@ export function isDescendant(maybeAncestor) {
 export function inType() {
   let path = this;
   while (path) {
-    for (let type of (arguments: Array)) {
+    for (const type of (arguments: Array)) {
       if (path.node.type === type) return true;
     }
     path = path.parentPath;
@@ -235,11 +235,11 @@ export function inType() {
  */
 
 export function inShadow(key?) {
-  let parentFn = this.isFunction() ? this : this.findParent((p) => p.isFunction());
+  const parentFn = this.isFunction() ? this : this.findParent((p) => p.isFunction());
   if (!parentFn) return;
 
   if (parentFn.isFunctionExpression() || parentFn.isFunctionDeclaration()) {
-    let shadow = parentFn.node.shadow;
+    const shadow = parentFn.node.shadow;
 
     // this is because sometimes we may have a `shadow` value of:
     //
diff --git a/packages/babel-traverse/src/path/comments.js b/packages/babel-traverse/src/path/comments.js
index b7f0ae1a07a6..8aec4d5fa5e6 100644
--- a/packages/babel-traverse/src/path/comments.js
+++ b/packages/babel-traverse/src/path/comments.js
@@ -8,11 +8,11 @@ export function shareCommentsWithSiblings() {
   // NOTE: this assumes numbered keys
   if (typeof this.key === "string") return;
 
-  let node = this.node;
+  const node = this.node;
   if (!node) return;
 
-  let trailing = node.trailingComments;
-  let leading  = node.leadingComments;
+  const trailing = node.trailingComments;
+  const leading  = node.leadingComments;
   if (!trailing && !leading) return;
 
   let prev = this.getSibling(this.key - 1);
@@ -39,10 +39,10 @@ export function addComment(type, content, line?) {
 export function addComments(type: string, comments: Array) {
   if (!comments) return;
 
-  let node = this.node;
+  const node = this.node;
   if (!node) return;
 
-  let key = `${type}Comments`;
+  const key = `${type}Comments`;
 
   if (node[key]) {
     node[key] = node[key].concat(comments);
diff --git a/packages/babel-traverse/src/path/context.js b/packages/babel-traverse/src/path/context.js
index 6710171c578e..c74cb356ddbd 100644
--- a/packages/babel-traverse/src/path/context.js
+++ b/packages/babel-traverse/src/path/context.js
@@ -3,7 +3,7 @@
 import traverse from "../index";
 
 export function call(key): boolean {
-  let opts = this.opts;
+  const opts = this.opts;
 
   this.debug(() => key);
 
@@ -21,13 +21,13 @@ export function call(key): boolean {
 export function _call(fns?: Array<Function>): boolean {
   if (!fns) return false;
 
-  for (let fn of fns) {
+  for (const fn of fns) {
     if (!fn) continue;
 
-    let node = this.node;
+    const node = this.node;
     if (!node) return true;
 
-    let ret = fn.call(this.state, this, this.state);
+    const ret = fn.call(this.state, this, this.state);
     if (ret) throw new Error(`Unexpected return value from visitor method ${fn}`);
 
     // node has been replaced, it will have been requeued
@@ -40,7 +40,7 @@ export function _call(fns?: Array<Function>): boolean {
 }
 
 export function isBlacklisted(): boolean {
-  let blacklist = this.opts.blacklist;
+  const blacklist = this.opts.blacklist;
   return blacklist && blacklist.indexOf(this.node.type) > -1;
 }
 
@@ -155,7 +155,7 @@ export function _resyncKey() {
       }
     }
   } else {
-    for (let key in this.container) {
+    for (const key in this.container) {
       if (this.container[key] === this.node) {
         return this.setKey(key);
       }
@@ -169,7 +169,7 @@ export function _resyncKey() {
 export function _resyncList() {
   if (!this.parent || !this.inList) return;
 
-  let newContainer = this.parent[this.listKey];
+  const newContainer = this.parent[this.listKey];
   if (this.container === newContainer) return;
 
   // container is out of sync. this is likely the result of it being reassigned
@@ -214,9 +214,9 @@ export function requeue(pathToQueue = this) {
   // TODO(loganfsmyth): This should be switched back to queue in parent contexts
   // automatically once #2892 and #4135 have been resolved. See #4140.
   // let contexts = this._getQueueContexts();
-  let contexts = this.contexts;
+  const contexts = this.contexts;
 
-  for (let context of contexts) {
+  for (const context of contexts) {
     context.maybeQueue(pathToQueue);
   }
 }
diff --git a/packages/babel-traverse/src/path/conversion.js b/packages/babel-traverse/src/path/conversion.js
index 27079529d71c..a9ff22fc640f 100644
--- a/packages/babel-traverse/src/path/conversion.js
+++ b/packages/babel-traverse/src/path/conversion.js
@@ -3,7 +3,7 @@
 import * as t from "babel-types";
 
 export function toComputedKey(): Object {
-  let node = this.node;
+  const node = this.node;
 
   let key;
   if (this.isMemberExpression()) {
@@ -31,7 +31,7 @@ export function arrowFunctionToShadowed() {
 
   this.ensureBlock();
 
-  let { node } = this;
+  const { node } = this;
   node.expression = false;
   node.type = "FunctionExpression";
   node.shadow = node.shadow || true;
diff --git a/packages/babel-traverse/src/path/evaluation.js b/packages/babel-traverse/src/path/evaluation.js
index 3b41b188c6ce..82c15a743ef7 100644
--- a/packages/babel-traverse/src/path/evaluation.js
+++ b/packages/babel-traverse/src/path/evaluation.js
@@ -29,7 +29,7 @@ const INVALID_METHODS = ["random"];
  */
 
 export function evaluateTruthy(): boolean {
-  let res = this.evaluate();
+  const res = this.evaluate();
   if (res.confident) return !!res.value;
 }
 
@@ -51,7 +51,7 @@ export function evaluateTruthy(): boolean {
 export function evaluate(): { confident: boolean; value: any } {
   let confident = true;
   let deoptPath: ?NodePath;
-  let seen = new Map;
+  const seen = new Map;
 
   function deopt(path) {
     if (!confident) return;
@@ -75,10 +75,10 @@ export function evaluate(): { confident: boolean; value: any } {
   //       a = g * this.foo
   //
   function evaluate(path) {
-    let { node } = path;
+    const { node } = path;
 
     if (seen.has(node)) {
-      let existing = seen.get(node);
+      const existing = seen.get(node);
       if (existing.resolved) {
         return existing.value;
       } else {
@@ -86,10 +86,10 @@ export function evaluate(): { confident: boolean; value: any } {
         return;
       }
     } else {
-      let item = { resolved: false };
+      const item = { resolved: false };
       seen.set(node, item);
 
-      let val = _evaluate(path);
+      const val = _evaluate(path);
       if (confident) {
         item.resolved = true;
         item.value = val;
@@ -101,10 +101,10 @@ export function evaluate(): { confident: boolean; value: any } {
   function _evaluate(path) {
     if (!confident) return;
 
-    let { node } = path;
+    const { node } = path;
 
     if (path.isSequenceExpression()) {
-      let exprs = path.get("expressions");
+      const exprs = path.get("expressions");
       return evaluate(exprs[exprs.length - 1]);
     }
 
@@ -120,9 +120,9 @@ export function evaluate(): { confident: boolean; value: any } {
       let str = "";
 
       let i = 0;
-      let exprs = path.get("expressions");
+      const exprs = path.get("expressions");
 
-      for (let elem of (node.quasis: Array<Object>)) {
+      for (const elem of (node.quasis: Array<Object>)) {
         // not confident, evaluated an expression we don't like
         if (!confident) break;
 
@@ -130,7 +130,7 @@ export function evaluate(): { confident: boolean; value: any } {
         str += elem.value.cooked;
 
         // add on interpolated expression if it's present
-        let expr = exprs[i++];
+        const expr = exprs[i++];
         if (expr) str += String(evaluate(expr));
       }
 
@@ -139,7 +139,7 @@ export function evaluate(): { confident: boolean; value: any } {
     }
 
     if (path.isConditionalExpression()) {
-      let testResult = evaluate(path.get("test"));
+      const testResult = evaluate(path.get("test"));
       if (!confident) return;
       if (testResult) {
         return evaluate(path.get("consequent"));
@@ -154,12 +154,12 @@ export function evaluate(): { confident: boolean; value: any } {
 
     // "foo".length
     if (path.isMemberExpression() && !path.parentPath.isCallExpression({ callee: node })) {
-      let property = path.get("property");
-      let object = path.get("object");
+      const property = path.get("property");
+      const object = path.get("object");
 
       if (object.isLiteral() && property.isIdentifier()) {
-        let value = object.node.value;
-        let type = typeof value;
+        const value = object.node.value;
+        const type = typeof value;
         if (type === "number" || type === "string") {
           return value[property.node.name];
         }
@@ -167,7 +167,7 @@ export function evaluate(): { confident: boolean; value: any } {
     }
 
     if (path.isReferencedIdentifier()) {
-      let binding = path.scope.getBinding(node.name);
+      const binding = path.scope.getBinding(node.name);
 
       if (binding && binding.constantViolations.length > 0) {
         return deopt(binding.path);
@@ -188,7 +188,7 @@ export function evaluate(): { confident: boolean; value: any } {
           return NaN;
         }
 
-        let resolved = path.resolve();
+        const resolved = path.resolve();
         if (resolved === path) {
           return deopt(path);
         } else {
@@ -203,12 +203,12 @@ export function evaluate(): { confident: boolean; value: any } {
         return undefined;
       }
 
-      let argument = path.get("argument");
+      const argument = path.get("argument");
       if (node.operator === "typeof" && (argument.isFunction() || argument.isClass())) {
         return "function";
       }
 
-      let arg = evaluate(argument);
+      const arg = evaluate(argument);
       if (!confident) return;
       switch (node.operator) {
         case "!": return !arg;
@@ -220,8 +220,8 @@ export function evaluate(): { confident: boolean; value: any } {
     }
 
     if (path.isArrayExpression()) {
-      let arr = [];
-      let elems: Array<NodePath> = path.get("elements");
+      const arr = [];
+      const elems: Array<NodePath> = path.get("elements");
       for (let elem of elems) {
         elem = elem.evaluate();
 
@@ -235,9 +235,9 @@ export function evaluate(): { confident: boolean; value: any } {
     }
 
     if (path.isObjectExpression()) {
-      let obj = {};
-      let props: Array<NodePath> = path.get("properties");
-      for (let prop of props) {
+      const obj = {};
+      const props: Array<NodePath> = path.get("properties");
+      for (const prop of props) {
         if (prop.isObjectMethod() || prop.isSpreadProperty()) {
           return deopt(prop);
         }
@@ -268,12 +268,12 @@ export function evaluate(): { confident: boolean; value: any } {
     if (path.isLogicalExpression()) {
       // If we are confident that one side of an && is false, or the left
       // side of an || is true, we can be confident about the entire expression
-      let wasConfident = confident;
-      let left = evaluate(path.get("left"));
-      let leftConfident = confident;
+      const wasConfident = confident;
+      const left = evaluate(path.get("left"));
+      const leftConfident = confident;
       confident = wasConfident;
-      let right = evaluate(path.get("right"));
-      let rightConfident = confident;
+      const right = evaluate(path.get("right"));
+      const rightConfident = confident;
       confident = leftConfident && rightConfident;
 
       switch (node.operator) {
@@ -300,9 +300,9 @@ export function evaluate(): { confident: boolean; value: any } {
     }
 
     if (path.isBinaryExpression()) {
-      let left = evaluate(path.get("left"));
+      const left = evaluate(path.get("left"));
       if (!confident) return;
-      let right = evaluate(path.get("right"));
+      const right = evaluate(path.get("right"));
       if (!confident) return;
 
       switch (node.operator) {
@@ -330,7 +330,7 @@ export function evaluate(): { confident: boolean; value: any } {
     }
 
     if (path.isCallExpression()) {
-      let callee = path.get("callee");
+      const callee = path.get("callee");
       let context;
       let func;
 
@@ -340,8 +340,8 @@ export function evaluate(): { confident: boolean; value: any } {
       }
 
       if (callee.isMemberExpression()) {
-        let object = callee.get("object");
-        let property = callee.get("property");
+        const object = callee.get("object");
+        const property = callee.get("property");
 
         // Math.min(1, 2)
         if (object.isIdentifier() && property.isIdentifier() && VALID_CALLEES.indexOf(object.node.name) >= 0 && INVALID_METHODS.indexOf(property.node.name) < 0) {
@@ -351,7 +351,7 @@ export function evaluate(): { confident: boolean; value: any } {
 
         // "abc".charCodeAt(4)
         if (object.isLiteral() && property.isIdentifier()) {
-          let type = typeof object.node.value;
+          const type = typeof object.node.value;
           if (type === "string" || type === "number") {
             context = object.node.value;
             func = context[property.node.name];
@@ -360,7 +360,7 @@ export function evaluate(): { confident: boolean; value: any } {
       }
 
       if (func) {
-        let args = path.get("arguments").map(evaluate);
+        const args = path.get("arguments").map(evaluate);
         if (!confident) return;
 
         return func.apply(context, args);
diff --git a/packages/babel-traverse/src/path/family.js b/packages/babel-traverse/src/path/family.js
index f7a630d0a79c..d17ae8aed709 100644
--- a/packages/babel-traverse/src/path/family.js
+++ b/packages/babel-traverse/src/path/family.js
@@ -33,7 +33,7 @@ export function getOpposite() {
 export function getCompletionRecords(): Array {
   let paths = [];
 
-  let add = function (path) {
+  const add = function (path) {
     if (path) paths = paths.concat(path.getCompletionRecords());
   };
 
@@ -69,7 +69,7 @@ export function getSibling(key) {
 
 export function get(key: string, context?: boolean | TraversalContext): NodePath {
   if (context === true) context = this.context;
-  let parts = key.split(".");
+  const parts = key.split(".");
   if (parts.length === 1) { // "foo"
     return this._getKey(key, context);
   } else { // "foo.bar"
@@ -78,8 +78,8 @@ export function get(key: string, context?: boolean | TraversalContext): NodePath
 }
 
 export function _getKey(key, context?) {
-  let node      = this.node;
-  let container = node[key];
+  const node      = this.node;
+  const container = node[key];
 
   if (Array.isArray(container)) {
     // requested a container so give them all the paths
@@ -104,7 +104,7 @@ export function _getKey(key, context?) {
 
 export function _getPattern(parts, context) {
   let path = this;
-  for (let part of (parts: Array)) {
+  for (const part of (parts: Array)) {
     if (part === ".") {
       path = path.parentPath;
     } else {
@@ -130,20 +130,20 @@ export function getOuterBindingIdentifiers(duplicates?) {
 // path.getBindingIdentifiers returns nodes where the following re-implementation
 // returns paths
 export function getBindingIdentifierPaths(duplicates = false, outerOnly = false) {
-  let path = this;
+  const path = this;
   let search = [].concat(path);
-  let ids = Object.create(null);
+  const ids = Object.create(null);
 
   while (search.length) {
-    let id = search.shift();
+    const id = search.shift();
     if (!id) continue;
     if (!id.node) continue;
 
-    let keys = t.getBindingIdentifiers.keys[id.node.type];
+    const keys = t.getBindingIdentifiers.keys[id.node.type];
 
     if (id.isIdentifier()) {
       if (duplicates) {
-        let _ids = ids[id.node.name] = ids[id.node.name] || [];
+        const _ids = ids[id.node.name] = ids[id.node.name] || [];
         _ids.push(id);
       } else {
         ids[id.node.name] = id;
@@ -171,8 +171,8 @@ export function getBindingIdentifierPaths(duplicates = false, outerOnly = false)
 
     if (keys) {
       for (let i = 0; i < keys.length; i++) {
-        let key = keys[i];
-        let child = id.get(key);
+        const key = keys[i];
+        const child = id.get(key);
         if (Array.isArray(child) || child.node) {
           search = search.concat(child);
         }
diff --git a/packages/babel-traverse/src/path/index.js b/packages/babel-traverse/src/path/index.js
index 1b5b58919018..853a735bb8bb 100644
--- a/packages/babel-traverse/src/path/index.js
+++ b/packages/babel-traverse/src/path/index.js
@@ -11,7 +11,7 @@ import Scope from "../scope";
 import * as t from "babel-types";
 import { path as pathCache } from "../cache";
 
-let debug = buildDebug("babel");
+const debug = buildDebug("babel");
 
 export default class NodePath {
   constructor(hub: Hub, parent: Object) {
@@ -67,9 +67,9 @@ export default class NodePath {
 
     invariant(parent, "To get a node path the parent needs to exist");
 
-    let targetNode = container[key];
+    const targetNode = container[key];
 
-    let paths = pathCache.get(parent) || [];
+    const paths = pathCache.get(parent) || [];
     if (!pathCache.has(parent)) {
       pathCache.set(parent, paths);
     }
@@ -77,7 +77,7 @@ export default class NodePath {
     let path;
 
     for (let i = 0; i < paths.length; i++) {
-      let pathCheck = paths[i];
+      const pathCheck = paths[i];
       if (pathCheck.node === targetNode) {
         path = pathCheck;
         break;
@@ -137,7 +137,7 @@ export default class NodePath {
   }
 
   getPathLocation(): string {
-    let parts = [];
+    const parts = [];
     let path = this;
     do {
       let key = path.key;
@@ -165,8 +165,8 @@ assign(NodePath.prototype, require("./modification"));
 assign(NodePath.prototype, require("./family"));
 assign(NodePath.prototype, require("./comments"));
 
-for (let type of (t.TYPES: Array<string>)) {
-  let typeKey = `is${type}`;
+for (const type of (t.TYPES: Array<string>)) {
+  const typeKey = `is${type}`;
   NodePath.prototype[typeKey] = function (opts) {
     return t[typeKey](this.node, opts);
   };
@@ -178,11 +178,11 @@ for (let type of (t.TYPES: Array<string>)) {
   };
 }
 
-for (let type in virtualTypes) {
+for (const type in virtualTypes) {
   if (type[0] === "_") continue;
   if (t.TYPES.indexOf(type) < 0) t.TYPES.push(type);
 
-  let virtualType = virtualTypes[type];
+  const virtualType = virtualTypes[type];
 
   NodePath.prototype[`is${type}`] = function (opts) {
     return virtualType.checkPath(this, opts);
diff --git a/packages/babel-traverse/src/path/inference/index.js b/packages/babel-traverse/src/path/inference/index.js
index 55a52911a655..976d327082b9 100644
--- a/packages/babel-traverse/src/path/inference/index.js
+++ b/packages/babel-traverse/src/path/inference/index.js
@@ -19,13 +19,13 @@ export function getTypeAnnotation(): Object {
  */
 
 export function _getTypeAnnotation(): ?Object {
-  let node = this.node;
+  const node = this.node;
 
   if (!node) {
     // handle initializerless variables, add in checks for loop initializers too
     if (this.key === "init" && this.parentPath.isVariableDeclarator()) {
-      let declar       = this.parentPath.parentPath;
-      let declarParent = declar.parentPath;
+      const declar       = this.parentPath.parentPath;
+      const declarParent = declar.parentPath;
 
       // for (let NODE in bar) {}
       if (declar.key === "left" && declarParent.isForInStatement()) {
@@ -87,11 +87,11 @@ function _isBaseType(baseName: string, type?, soft?): boolean {
 }
 
 export function couldBeBaseType(name: string): boolean {
-  let type = this.getTypeAnnotation();
+  const type = this.getTypeAnnotation();
   if (t.isAnyTypeAnnotation(type)) return true;
 
   if (t.isUnionTypeAnnotation(type)) {
-    for (let type2 of (type.types: Array<Object>)) {
+    for (const type2 of (type.types: Array<Object>)) {
       if (t.isAnyTypeAnnotation(type2) || _isBaseType(name, type2, true)) {
         return true;
       }
@@ -103,7 +103,7 @@ export function couldBeBaseType(name: string): boolean {
 }
 
 export function baseTypeStrictlyMatches(right: NodePath) {
-  let left = this.getTypeAnnotation();
+  const left = this.getTypeAnnotation();
   right = right.getTypeAnnotation();
 
   if (!t.isAnyTypeAnnotation(left) && t.isFlowBaseAnnotation(left)) {
@@ -112,6 +112,6 @@ export function baseTypeStrictlyMatches(right: NodePath) {
 }
 
 export function isGenericType(genericName: string): boolean {
-  let type = this.getTypeAnnotation();
+  const type = this.getTypeAnnotation();
   return t.isGenericTypeAnnotation(type) && t.isIdentifier(type.id, { name: genericName });
 }
diff --git a/packages/babel-traverse/src/path/inference/inferer-reference.js b/packages/babel-traverse/src/path/inference/inferer-reference.js
index 4b1d8e58a926..16effb09430d 100644
--- a/packages/babel-traverse/src/path/inference/inferer-reference.js
+++ b/packages/babel-traverse/src/path/inference/inferer-reference.js
@@ -6,7 +6,7 @@ export default function (node: Object) {
 
   // check if a binding exists of this value and if so then return a union type of all
   // possible types that the binding could be
-  let binding = this.scope.getBinding(node.name);
+  const binding = this.scope.getBinding(node.name);
   if (binding) {
     if (binding.identifier.typeAnnotation) {
       return binding.identifier.typeAnnotation;
@@ -26,17 +26,17 @@ export default function (node: Object) {
 }
 
 function getTypeAnnotationBindingConstantViolations(path, name) {
-  let binding = path.scope.getBinding(name);
+  const binding = path.scope.getBinding(name);
 
-  let types = [];
+  const types = [];
   path.typeAnnotation = t.unionTypeAnnotation(types);
 
-  let functionConstantViolations = [];
+  const functionConstantViolations = [];
   let constantViolations = getConstantViolationsBefore(binding, path, functionConstantViolations);
 
-  let testType = getConditionalAnnotation(path, name);
+  const testType = getConditionalAnnotation(path, name);
   if (testType) {
-    let testConstantViolations = getConstantViolationsBefore(binding, testType.ifStatement);
+    const testConstantViolations = getConstantViolationsBefore(binding, testType.ifStatement);
 
     // remove constant violations observed before the IfStatement
     constantViolations = constantViolations.filter((path) => testConstantViolations.indexOf(path) < 0);
@@ -76,7 +76,7 @@ function getTypeAnnotationBindingConstantViolations(path, name) {
     constantViolations = constantViolations.concat(functionConstantViolations);
 
     // push on inferred types of violated paths
-    for (let violation of (constantViolations: Array<NodePath>)) {
+    for (const violation of (constantViolations: Array<NodePath>)) {
       types.push(violation.getTypeAnnotation());
     }
   }
@@ -87,21 +87,21 @@ function getTypeAnnotationBindingConstantViolations(path, name) {
 }
 
 function getConstantViolationsBefore(binding, path, functions) {
-  let violations = binding.constantViolations.slice();
+  const violations = binding.constantViolations.slice();
   violations.unshift(binding.path);
   return violations.filter((violation) => {
     violation = violation.resolve();
-    let status = violation._guessExecutionStatusRelativeTo(path);
+    const status = violation._guessExecutionStatusRelativeTo(path);
     if (functions && status === "function") functions.push(violation);
     return status === "before";
   });
 }
 
 function inferAnnotationFromBinaryExpression(name, path) {
-  let operator = path.node.operator;
+  const operator = path.node.operator;
 
-  let right = path.get("right").resolve();
-  let left  = path.get("left").resolve();
+  const right = path.get("right").resolve();
+  const left  = path.get("left").resolve();
 
   let target;
   if (left.isIdentifier({ name })) {
@@ -138,7 +138,7 @@ function inferAnnotationFromBinaryExpression(name, path) {
   if (!typePath.isLiteral()) return;
 
   // and that it's a string so we can infer it
-  let typeValue = typePath.node.value;
+  const typeValue = typePath.node.value;
   if (typeof typeValue !== "string") return;
 
   // and that the argument of the typeof path references us!
@@ -164,15 +164,15 @@ function getParentConditionalPath(path) {
 }
 
 function getConditionalAnnotation(path, name) {
-  let ifStatement = getParentConditionalPath(path);
+  const ifStatement = getParentConditionalPath(path);
   if (!ifStatement) return;
 
-  let test  = ifStatement.get("test");
-  let paths = [test];
-  let types = [];
+  const test  = ifStatement.get("test");
+  const paths = [test];
+  const types = [];
 
   do {
-    let path = paths.shift().resolve();
+    const path = paths.shift().resolve();
 
     if (path.isLogicalExpression()) {
       paths.push(path.get("left"));
@@ -180,7 +180,7 @@ function getConditionalAnnotation(path, name) {
     }
 
     if (path.isBinaryExpression()) {
-      let type = inferAnnotationFromBinaryExpression(name, path);
+      const type = inferAnnotationFromBinaryExpression(name, path);
       if (type) types.push(type);
     }
   } while (paths.length);
diff --git a/packages/babel-traverse/src/path/inference/inferers.js b/packages/babel-traverse/src/path/inference/inferers.js
index ec372f6825d7..be90de0cc0f8 100644
--- a/packages/babel-traverse/src/path/inference/inferers.js
+++ b/packages/babel-traverse/src/path/inference/inferers.js
@@ -3,7 +3,7 @@ import * as t from "babel-types";
 export { default as Identifier } from "./inferer-reference";
 
 export function VariableDeclarator() {
-  let id = this.get("id");
+  const id = this.get("id");
 
   if (id.isIdentifier()) {
     return this.get("init").getTypeAnnotation();
@@ -30,7 +30,7 @@ export function TemplateLiteral() {
 }
 
 export function UnaryExpression(node) {
-  let operator = node.operator;
+  const operator = node.operator;
 
   if (operator === "void") {
     return t.voidTypeAnnotation();
@@ -44,15 +44,15 @@ export function UnaryExpression(node) {
 }
 
 export function BinaryExpression(node) {
-  let operator = node.operator;
+  const operator = node.operator;
 
   if (t.NUMBER_BINARY_OPERATORS.indexOf(operator) >= 0) {
     return t.numberTypeAnnotation();
   } else if (t.BOOLEAN_BINARY_OPERATORS.indexOf(operator) >= 0) {
     return t.booleanTypeAnnotation();
   } else if (operator === "+") {
-    let right = this.get("right");
-    let left  = this.get("left");
+    const right = this.get("right");
+    const left  = this.get("left");
 
     if (left.isBaseType("number") && right.isBaseType("number")) {
       // both numbers so this will be a number
@@ -93,7 +93,7 @@ export function AssignmentExpression() {
 }
 
 export function UpdateExpression(node) {
-  let operator = node.operator;
+  const operator = node.operator;
   if (operator === "++" || operator === "--") {
     return t.numberTypeAnnotation();
   }
diff --git a/packages/babel-traverse/src/path/introspection.js b/packages/babel-traverse/src/path/introspection.js
index be7a2755c537..e8e54f18c01f 100644
--- a/packages/babel-traverse/src/path/introspection.js
+++ b/packages/babel-traverse/src/path/introspection.js
@@ -15,17 +15,17 @@ export function matchesPattern(pattern: string, allowPartial?: boolean): boolean
   // not a member expression
   if (!this.isMemberExpression()) return false;
 
-  let parts = pattern.split(".");
-  let search = [this.node];
+  const parts = pattern.split(".");
+  const search = [this.node];
   let i = 0;
 
   function matches(name) {
-    let part = parts[i];
+    const part = parts[i];
     return part === "*" || name === part;
   }
 
   while (search.length) {
-    let node = search.shift();
+    const node = search.shift();
 
     if (allowPartial && i === parts.length) {
       return true;
@@ -68,7 +68,7 @@ export function matchesPattern(pattern: string, allowPartial?: boolean): boolean
  */
 
 export function has(key): boolean {
-  let val = this.node && this.node[key];
+  const val = this.node && this.node[key];
   if (val && Array.isArray(val)) {
     return !!val.length;
   } else {
@@ -88,7 +88,7 @@ export function isStatic() {
  * Alias of `has`.
  */
 
-export let is = has;
+export const is = has;
 
 /**
  * Opposite of `has`.
@@ -160,7 +160,7 @@ export function isCompletionRecord(allowInsideFunction?) {
   let first = true;
 
   do {
-    let container = path.container;
+    const container = path.container;
 
     // we're in a function so can't be a completion record
     if (path.isFunction() && !first) {
@@ -199,11 +199,11 @@ export function isStatementOrBlock() {
 export function referencesImport(moduleSource, importName) {
   if (!this.isReferencedIdentifier()) return false;
 
-  let binding = this.scope.getBinding(this.node.name);
+  const binding = this.scope.getBinding(this.node.name);
   if (!binding || binding.kind !== "module") return false;
 
-  let path = binding.path;
-  let parent = path.parentPath;
+  const path = binding.path;
+  const parent = path.parentPath;
   if (!parent.isImportDeclaration()) return false;
 
   // check moduleSource
@@ -233,7 +233,7 @@ export function referencesImport(moduleSource, importName) {
  */
 
 export function getSource() {
-  let node = this.node;
+  const node = this.node;
   if (node.end) {
     return this.hub.file.code.slice(node.start, node.end);
   } else {
@@ -254,13 +254,13 @@ export function willIMaybeExecuteBefore(target) {
 
 export function _guessExecutionStatusRelativeTo(target) {
   // check if the two paths are in different functions, we can't track execution of these
-  let targetFuncParent = target.scope.getFunctionParent();
-  let selfFuncParent = this.scope.getFunctionParent();
+  const targetFuncParent = target.scope.getFunctionParent();
+  const selfFuncParent = this.scope.getFunctionParent();
 
   // here we check the `node` equality as sometimes we may have different paths for the
   // same node due to path thrashing
   if (targetFuncParent.node !== selfFuncParent.node) {
-    let status = this._guessExecutionStatusRelativeToDifferentFunctions(targetFuncParent);
+    const status = this._guessExecutionStatusRelativeToDifferentFunctions(targetFuncParent);
     if (status) {
       return status;
     } else {
@@ -268,17 +268,17 @@ export function _guessExecutionStatusRelativeTo(target) {
     }
   }
 
-  let targetPaths = target.getAncestry();
+  const targetPaths = target.getAncestry();
   if (targetPaths.indexOf(this) >= 0) return "after";
 
-  let selfPaths   = this.getAncestry();
+  const selfPaths   = this.getAncestry();
 
   // get ancestor where the branches intersect
   let commonPath;
   let targetIndex;
   let selfIndex;
   for (selfIndex = 0; selfIndex < selfPaths.length; selfIndex++) {
-    let selfPath = selfPaths[selfIndex];
+    const selfPath = selfPaths[selfIndex];
     targetIndex = targetPaths.indexOf(selfPath);
     if (targetIndex >= 0) {
       commonPath = selfPath;
@@ -290,8 +290,8 @@ export function _guessExecutionStatusRelativeTo(target) {
   }
 
   // get the relationship paths that associate these nodes to their common ancestor
-  let targetRelationship = targetPaths[targetIndex - 1];
-  let selfRelationship   = selfPaths[selfIndex - 1];
+  const targetRelationship = targetPaths[targetIndex - 1];
+  const selfRelationship   = selfPaths[selfIndex - 1];
   if (!targetRelationship || !selfRelationship) {
     return "before";
   }
@@ -302,28 +302,28 @@ export function _guessExecutionStatusRelativeTo(target) {
   }
 
   // otherwise we're associated by a parent node, check which key comes before the other
-  let targetKeyPosition = t.VISITOR_KEYS[targetRelationship.type].indexOf(targetRelationship.key);
-  let selfKeyPosition   = t.VISITOR_KEYS[selfRelationship.type].indexOf(selfRelationship.key);
+  const targetKeyPosition = t.VISITOR_KEYS[targetRelationship.type].indexOf(targetRelationship.key);
+  const selfKeyPosition   = t.VISITOR_KEYS[selfRelationship.type].indexOf(selfRelationship.key);
   return targetKeyPosition > selfKeyPosition ? "before" : "after";
 }
 
 export function _guessExecutionStatusRelativeToDifferentFunctions(targetFuncParent) {
-  let targetFuncPath = targetFuncParent.path;
+  const targetFuncPath = targetFuncParent.path;
   if (!targetFuncPath.isFunctionDeclaration()) return;
 
   // so we're in a completely different function, if this is a function declaration
   // then we can be a bit smarter and handle cases where the function is either
   // a. not called at all (part of an export)
   // b. called directly
-  let binding = targetFuncPath.scope.getBinding(targetFuncPath.node.id.name);
+  const binding = targetFuncPath.scope.getBinding(targetFuncPath.node.id.name);
 
   // no references!
   if (!binding.references) return "before";
 
-  let referencePaths: Array<NodePath> = binding.referencePaths;
+  const referencePaths: Array<NodePath> = binding.referencePaths;
 
   // verify that all of the references are calls
-  for (let path of referencePaths) {
+  for (const path of referencePaths) {
     if (path.key !== "callee" || !path.parentPath.isCallExpression()) {
       return;
     }
@@ -332,13 +332,13 @@ export function _guessExecutionStatusRelativeToDifferentFunctions(targetFuncPare
   let allStatus;
 
   // verify that all the calls have the same execution status
-  for (let path of referencePaths) {
+  for (const path of referencePaths) {
     // if a reference is a child of the function we're checking against then we can
     // safelty ignore it
-    let childOfFunction = !!path.find((path) => path.node === targetFuncPath.node);
+    const childOfFunction = !!path.find((path) => path.node === targetFuncPath.node);
     if (childOfFunction) continue;
 
-    let status = this._guessExecutionStatusRelativeTo(path);
+    const status = this._guessExecutionStatusRelativeTo(path);
 
     if (allStatus) {
       if (allStatus !== status) return;
@@ -374,7 +374,7 @@ export function _resolve(dangerous?, resolved?): ?NodePath {
       // otherwise it's a request for a pattern and that's a bit more tricky
     }
   } else if (this.isReferencedIdentifier()) {
-    let binding = this.scope.getBinding(this.node.name);
+    const binding = this.scope.getBinding(this.node.name);
     if (!binding) return;
 
     // reassigned so we can't really resolve it
@@ -384,7 +384,7 @@ export function _resolve(dangerous?, resolved?): ?NodePath {
     if (binding.kind === "module") return;
 
     if (binding.path !== this) {
-      let ret = binding.path.resolve(dangerous, resolved);
+      const ret = binding.path.resolve(dangerous, resolved);
       // If the identifier resolves to parent node then we can't really resolve it.
       if (this.find((parent) => parent.node === ret.node)) return;
       return ret;
@@ -395,19 +395,19 @@ export function _resolve(dangerous?, resolved?): ?NodePath {
     // this is dangerous, as non-direct target assignments will mutate it's state
     // making this resolution inaccurate
 
-    let targetKey = this.toComputedKey();
+    const targetKey = this.toComputedKey();
     if (!t.isLiteral(targetKey)) return;
 
-    let targetName = targetKey.value;
+    const targetName = targetKey.value;
 
-    let target = this.get("object").resolve(dangerous, resolved);
+    const target = this.get("object").resolve(dangerous, resolved);
 
     if (target.isObjectExpression()) {
-      let props = target.get("properties");
-      for (let prop of (props: Array)) {
+      const props = target.get("properties");
+      for (const prop of (props: Array)) {
         if (!prop.isProperty()) continue;
 
-        let key = prop.get("key");
+        const key = prop.get("key");
 
         // { foo: obj }
         let match = prop.isnt("computed") && key.isIdentifier({ name: targetName });
@@ -418,8 +418,8 @@ export function _resolve(dangerous?, resolved?): ?NodePath {
         if (match) return prop.get("value").resolve(dangerous, resolved);
       }
     } else if (target.isArrayExpression() && !isNaN(+targetName)) {
-      let elems = target.get("elements");
-      let elem = elems[targetName];
+      const elems = target.get("elements");
+      const elem = elems[targetName];
       if (elem) return elem.resolve(dangerous, resolved);
     }
   }
diff --git a/packages/babel-traverse/src/path/lib/hoister.js b/packages/babel-traverse/src/path/lib/hoister.js
index 3776b2d30e02..8a36031f1e79 100644
--- a/packages/babel-traverse/src/path/lib/hoister.js
+++ b/packages/babel-traverse/src/path/lib/hoister.js
@@ -1,14 +1,14 @@
 import { react } from "babel-types";
 import * as t from "babel-types";
 
-let referenceVisitor = {
+const referenceVisitor = {
   ReferencedIdentifier(path, state) {
     if (path.isJSXIdentifier() && react.isCompatTag(path.node.name)) {
       return;
     }
 
     // direct references that we need to track to hoist this to the highest scope we can
-    let binding = path.scope.getBinding(path.node.name);
+    const binding = path.scope.getBinding(path.node.name);
     if (!binding) return;
 
     // this binding isn't accessible from the parent scope so we can safely ignore it
@@ -18,7 +18,7 @@ let referenceVisitor = {
     if (binding.constant) {
       state.bindings[path.node.name] = binding;
     } else {
-      for (let violationPath of (binding.constantViolations: Array)) {
+      for (const violationPath of (binding.constantViolations: Array)) {
         state.breakOnScopePaths = state.breakOnScopePaths.concat(violationPath.getAncestry());
       }
     }
@@ -35,8 +35,8 @@ export default class PathHoister {
   }
 
   isCompatibleScope(scope) {
-    for (let key in this.bindings) {
-      let binding = this.bindings[key];
+    for (const key in this.bindings) {
+      const binding = this.bindings[key];
       if (!scope.bindingIdentifierEquals(key, binding.identifier)) {
         return false;
       }
@@ -61,7 +61,7 @@ export default class PathHoister {
   }
 
   getAttachmentPath() {
-    let path = this._getAttachmentPath();
+    const path = this._getAttachmentPath();
     if (!path) return;
 
     let targetScope = path.scope;
@@ -73,11 +73,11 @@ export default class PathHoister {
 
     // avoid hoisting to a scope that contains bindings that are executed after our attachment path
     if (targetScope.path.isProgram() || targetScope.path.isFunction()) {
-      for (let name in this.bindings) {
+      for (const name in this.bindings) {
         // check binding is a direct child of this paths scope
         if (!targetScope.hasOwnBinding(name)) continue;
 
-        let binding = this.bindings[name];
+        const binding = this.bindings[name];
 
         // allow parameter references
         if (binding.kind === "param") continue;
@@ -91,9 +91,9 @@ export default class PathHoister {
   }
 
   _getAttachmentPath() {
-    let scopes = this.scopes;
+    const scopes = this.scopes;
 
-    let scope = scopes.pop();
+    const scope = scopes.pop();
     if (!scope) return;
 
     if (scope.path.isFunction()) {
@@ -113,7 +113,7 @@ export default class PathHoister {
   }
 
   getNextScopeAttachmentParent() {
-    let scope = this.scopes.pop();
+    const scope = this.scopes.pop();
     if (scope) return this.getAttachmentParentForPath(scope.path);
   }
 
@@ -127,17 +127,17 @@ export default class PathHoister {
   }
 
   hasOwnParamBindings(scope) {
-    for (let name in this.bindings) {
+    for (const name in this.bindings) {
       if (!scope.hasOwnBinding(name)) continue;
 
-      let binding = this.bindings[name];
+      const binding = this.bindings[name];
       if (binding.kind === "param") return true;
     }
     return false;
   }
 
   run() {
-    let node = this.path.node;
+    const node = this.path.node;
     if (node._hoisted) return;
     node._hoisted = true;
 
@@ -145,7 +145,7 @@ export default class PathHoister {
 
     this.getCompatibleScopes();
 
-    let attachTo = this.getAttachmentPath();
+    const attachTo = this.getAttachmentPath();
     if (!attachTo) return;
 
     // don't bother hoisting to the same function as this will cause multiple branches to be evaluated more than once leading to a bad optimisation
@@ -153,13 +153,13 @@ export default class PathHoister {
 
     // generate declaration and insert it to our point
     let uid = attachTo.scope.generateUidIdentifier("ref");
-    let declarator = t.variableDeclarator(uid, this.path.node);
+    const declarator = t.variableDeclarator(uid, this.path.node);
 
     attachTo.insertBefore([
       attachTo.isVariableDeclarator() ? declarator : t.variableDeclaration("var", [declarator])
     ]);
 
-    let parent = this.path.parentPath;
+    const parent = this.path.parentPath;
     if (parent.isJSXElement() && this.path.container === parent.node.children) {
       // turning the `span` in `<div><span /></div>` to an expression so we need to wrap it with
       // an expression container
diff --git a/packages/babel-traverse/src/path/lib/removal-hooks.js b/packages/babel-traverse/src/path/lib/removal-hooks.js
index 07cc98ab46db..0678d1da28e0 100644
--- a/packages/babel-traverse/src/path/lib/removal-hooks.js
+++ b/packages/babel-traverse/src/path/lib/removal-hooks.js
@@ -4,7 +4,7 @@
  * Pre hooks should be used for either rejecting removal or delegating removal
  */
 
-export let hooks = [
+export const hooks = [
   function (self, parent) {
     let removeParent = false;
 
diff --git a/packages/babel-traverse/src/path/lib/virtual-types.js b/packages/babel-traverse/src/path/lib/virtual-types.js
index fec4dea2effd..07ba90714435 100644
--- a/packages/babel-traverse/src/path/lib/virtual-types.js
+++ b/packages/babel-traverse/src/path/lib/virtual-types.js
@@ -2,7 +2,7 @@ import type NodePath from "../index";
 import { react } from "babel-types";
 import * as t from "babel-types";
 
-export let ReferencedIdentifier = {
+export const ReferencedIdentifier = {
   types: ["Identifier", "JSXIdentifier"],
   checkPath({ node, parent }: NodePath, opts?: Object): boolean {
     if (!t.isIdentifier(node, opts) && !t.isJSXMemberExpression(parent, opts)) {
@@ -19,21 +19,21 @@ export let ReferencedIdentifier = {
   }
 };
 
-export let ReferencedMemberExpression = {
+export const ReferencedMemberExpression = {
   types: ["MemberExpression"],
   checkPath({ node, parent }) {
     return t.isMemberExpression(node) && t.isReferenced(node, parent);
   }
 };
 
-export let BindingIdentifier = {
+export const BindingIdentifier = {
   types: ["Identifier"],
   checkPath({ node, parent }: NodePath): boolean {
     return t.isIdentifier(node) && t.isBinding(node, parent);
   }
 };
 
-export let Statement = {
+export const Statement = {
   types: ["Statement"],
   checkPath({ node, parent }: NodePath): boolean {
     if (t.isStatement(node)) {
@@ -49,7 +49,7 @@ export let Statement = {
   }
 };
 
-export let Expression = {
+export const Expression = {
   types: ["Expression"],
   checkPath(path: NodePath): boolean {
     if (path.isIdentifier()) {
@@ -60,51 +60,51 @@ export let Expression = {
   }
 };
 
-export let Scope = {
+export const Scope = {
   types: ["Scopable"],
   checkPath(path) {
     return t.isScope(path.node, path.parent);
   }
 };
 
-export let Referenced = {
+export const Referenced = {
   checkPath(path: NodePath): boolean {
     return t.isReferenced(path.node, path.parent);
   }
 };
 
-export let BlockScoped = {
+export const BlockScoped = {
   checkPath(path: NodePath): boolean {
     return t.isBlockScoped(path.node);
   }
 };
 
-export let Var = {
+export const Var = {
   types: ["VariableDeclaration"],
   checkPath(path: NodePath): boolean {
     return t.isVar(path.node);
   }
 };
 
-export let User = {
+export const User = {
   checkPath(path: NodePath): boolean {
     return path.node && !!path.node.loc;
   }
 };
 
-export let Generated = {
+export const Generated = {
   checkPath(path: NodePath): boolean {
     return !path.isUser();
   }
 };
 
-export let Pure = {
+export const Pure = {
   checkPath(path: NodePath, opts?): boolean {
     return path.scope.isPure(path.node, opts);
   }
 };
 
-export let Flow = {
+export const Flow = {
   types: ["Flow", "ImportDeclaration", "ExportDeclaration", "ImportSpecifier"],
   checkPath({ node }: NodePath): boolean {
     if (t.isFlow(node)) {
diff --git a/packages/babel-traverse/src/path/modification.js b/packages/babel-traverse/src/path/modification.js
index cb4568240c6f..704014c16eeb 100644
--- a/packages/babel-traverse/src/path/modification.js
+++ b/packages/babel-traverse/src/path/modification.js
@@ -38,15 +38,15 @@ export function insertBefore(nodes) {
 export function _containerInsert(from, nodes) {
   this.updateSiblingKeys(from, nodes.length);
 
-  let paths = [];
+  const paths = [];
 
   for (let i = 0; i < nodes.length; i++) {
-    let to = from + i;
-    let node = nodes[i];
+    const to = from + i;
+    const node = nodes[i];
     this.container.splice(to, 0, node);
 
     if (this.context) {
-      let path = this.context.create(this.parent, this.container, to, this.listKey);
+      const path = this.context.create(this.parent, this.container, to, this.listKey);
 
       // While this path may have a context, there is currently no guarantee that the context
       // will be the active context, because `popContext` may leave a final context in place.
@@ -64,13 +64,13 @@ export function _containerInsert(from, nodes) {
     }
   }
 
-  let contexts = this._getQueueContexts();
+  const contexts = this._getQueueContexts();
 
-  for (let path of paths) {
+  for (const path of paths) {
     path.setScope();
     path.debug(() => "Inserted.");
 
-    for (let context of contexts) {
+    for (const context of contexts) {
       context.maybeQueue(path, true);
     }
   }
@@ -87,8 +87,8 @@ export function _containerInsertAfter(nodes) {
 }
 
 export function _maybePopFromStatements(nodes) {
-  let last = nodes[nodes.length - 1];
-  let isIdentifier = t.isIdentifier(last) || (t.isExpressionStatement(last) && t.isIdentifier(last.expression));
+  const last = nodes[nodes.length - 1];
+  const isIdentifier = t.isIdentifier(last) || (t.isExpressionStatement(last) && t.isIdentifier(last.expression));
 
   if (isIdentifier && !this.isCompletionRecord()) {
     nodes.pop();
@@ -109,7 +109,7 @@ export function insertAfter(nodes) {
     return this.parentPath.insertAfter(nodes);
   } else if (this.isNodeType("Expression") || (this.parentPath.isForStatement() && this.key === "init")) {
     if (this.node) {
-      let temp = this.scope.generateDeclaredUidIdentifier();
+      const temp = this.scope.generateDeclaredUidIdentifier();
       nodes.unshift(t.expressionStatement(t.assignmentExpression("=", temp, this.node)));
       nodes.push(t.expressionStatement(temp));
     }
@@ -136,9 +136,9 @@ export function insertAfter(nodes) {
 export function updateSiblingKeys(fromIndex, incrementBy) {
   if (!this.parent) return;
 
-  let paths = pathCache.get(this.parent);
+  const paths = pathCache.get(this.parent);
   for (let i = 0; i < paths.length; i++) {
-    let path = paths[i];
+    const path = paths[i];
     if (path.key >= fromIndex) {
       path.key += incrementBy;
     }
@@ -155,7 +155,7 @@ export function _verifyNodeList(nodes) {
   }
 
   for (let i = 0; i < nodes.length; i++) {
-    let node = nodes[i];
+    const node = nodes[i];
     let msg;
 
     if (!node) {
@@ -169,7 +169,7 @@ export function _verifyNodeList(nodes) {
     }
 
     if (msg) {
-      let type = Array.isArray(node) ? "array" : typeof node;
+      const type = Array.isArray(node) ? "array" : typeof node;
       throw new Error(`Node list ${msg} with the index of ${i} and type of ${type}`);
     }
   }
@@ -184,7 +184,7 @@ export function unshiftContainer(listKey, nodes) {
 
   // get the first path and insert our nodes before it, if it doesn't exist then it
   // doesn't matter, our nodes will be inserted anyway
-  let path = NodePath.get({
+  const path = NodePath.get({
     parentPath: this,
     parent: this.node,
     container: this.node[listKey],
@@ -203,8 +203,8 @@ export function pushContainer(listKey, nodes) {
   // get an invisible path that represents the last node + 1 and replace it with our
   // nodes, effectively inlining it
 
-  let container = this.node[listKey];
-  let path = NodePath.get({
+  const container = this.node[listKey];
+  const path = NodePath.get({
     parentPath: this,
     parent: this.node,
     container: container,
@@ -221,6 +221,6 @@ export function pushContainer(listKey, nodes) {
  */
 
 export function hoist(scope = this.scope) {
-  let hoister = new PathHoister(this, scope);
+  const hoister = new PathHoister(this, scope);
   return hoister.run();
 }
diff --git a/packages/babel-traverse/src/path/removal.js b/packages/babel-traverse/src/path/removal.js
index f7686fce6b41..76556e92eb6a 100644
--- a/packages/babel-traverse/src/path/removal.js
+++ b/packages/babel-traverse/src/path/removal.js
@@ -18,7 +18,7 @@ export function remove() {
 }
 
 export function _callRemovalHooks() {
-  for (let fn of (hooks: Array<Function>)) {
+  for (const fn of (hooks: Array<Function>)) {
     if (fn(this, this.parentPath)) return true;
   }
 }
diff --git a/packages/babel-traverse/src/path/replacement.js b/packages/babel-traverse/src/path/replacement.js
index 8ca15a4c094e..08df6ce16c94 100644
--- a/packages/babel-traverse/src/path/replacement.js
+++ b/packages/babel-traverse/src/path/replacement.js
@@ -7,7 +7,7 @@ import NodePath from "./index";
 import { parse } from "babylon";
 import * as t from "babel-types";
 
-let hoistVariablesVisitor = {
+const hoistVariablesVisitor = {
   Function(path) {
     path.skip();
   },
@@ -15,14 +15,14 @@ let hoistVariablesVisitor = {
   VariableDeclaration(path) {
     if (path.node.kind !== "var") return;
 
-    let bindings = path.getBindingIdentifiers();
-    for (let key in bindings) {
+    const bindings = path.getBindingIdentifiers();
+    for (const key in bindings) {
       path.scope.push({ id: bindings[key] });
     }
 
-    let exprs = [];
+    const exprs = [];
 
-    for (let declar of (path.node.declarations: Array<Object>)) {
+    for (const declar of (path.node.declarations: Array<Object>)) {
       if (declar.init) {
         exprs.push(t.expressionStatement(
           t.assignmentExpression("=", declar.id, declar.init)
@@ -73,7 +73,7 @@ export function replaceWithSourceString(replacement) {
     replacement = `(${replacement})`;
     replacement = parse(replacement);
   } catch (err) {
-    let loc = err.loc;
+    const loc = err.loc;
     if (loc) {
       err.message += " - make sure this is an expression.";
       err.message += "\n" + codeFrame(replacement, loc.line, loc.column + 1);
@@ -135,7 +135,7 @@ export function replaceWith(replacement) {
     }
   }
 
-  let oldNode = this.node;
+  const oldNode = this.node;
   if (oldNode) {
     t.inheritsComments(replacement, oldNode);
     t.removeComments(oldNode);
@@ -181,10 +181,10 @@ export function _replaceWith(node) {
 export function replaceExpressionWithStatements(nodes: Array<Object>) {
   this.resync();
 
-  let toSequenceExpression = t.toSequenceExpression(nodes, this.scope);
+  const toSequenceExpression = t.toSequenceExpression(nodes, this.scope);
 
   if (t.isSequenceExpression(toSequenceExpression)) {
-    let exprs = toSequenceExpression.expressions;
+    const exprs = toSequenceExpression.expressions;
 
     if (exprs.length >= 2 && this.parentPath.isExpressionStatement()) {
       this._maybePopFromStatements(exprs);
@@ -199,22 +199,22 @@ export function replaceExpressionWithStatements(nodes: Array<Object>) {
   } else if (toSequenceExpression) {
     this.replaceWith(toSequenceExpression);
   } else {
-    let container = t.functionExpression(null, [], t.blockStatement(nodes));
+    const container = t.functionExpression(null, [], t.blockStatement(nodes));
     container.shadow = true;
 
     this.replaceWith(t.callExpression(container, []));
     this.traverse(hoistVariablesVisitor);
 
     // add implicit returns to all ending expression statements
-    let completionRecords: Array<NodePath> = this.get("callee").getCompletionRecords();
-    for (let path of completionRecords) {
+    const completionRecords: Array<NodePath> = this.get("callee").getCompletionRecords();
+    for (const path of completionRecords) {
       if (!path.isExpressionStatement()) continue;
 
-      let loop = path.findParent((path) => path.isLoop());
+      const loop = path.findParent((path) => path.isLoop());
       if (loop) {
-        let callee = this.get("callee");
+        const callee = this.get("callee");
 
-        let uid = callee.scope.generateDeclaredUidIdentifier("ret");
+        const uid = callee.scope.generateDeclaredUidIdentifier("ret");
         callee.get("body").pushContainer("body", t.returnStatement(uid));
 
         path.get("expression").replaceWith(
diff --git a/packages/babel-traverse/src/scope/index.js b/packages/babel-traverse/src/scope/index.js
index 83e80df926a8..b7b3388b8ddb 100644
--- a/packages/babel-traverse/src/scope/index.js
+++ b/packages/babel-traverse/src/scope/index.js
@@ -24,9 +24,9 @@ let _crawlCallsCount = 0;
  */
 
 function getCache(path, parentScope, self) {
-  let scopes: Array<Scope> = scopeCache.get(path.node) || [];
+  const scopes: Array<Scope> = scopeCache.get(path.node) || [];
 
-  for (let scope of scopes) {
+  for (const scope of scopes) {
     if (scope.parent === parentScope && scope.path === path) return scope;
   }
 
@@ -43,7 +43,7 @@ function gatherNodeParts(node: Object, parts: Array) {
     if (node.source) {
       gatherNodeParts(node.source, parts);
     } else if (node.specifiers && node.specifiers.length) {
-      for (let specifier of (node.specifiers: Array)) {
+      for (const specifier of (node.specifiers: Array)) {
         gatherNodeParts(specifier, parts);
       }
     } else if (node.declaration) {
@@ -61,7 +61,7 @@ function gatherNodeParts(node: Object, parts: Array) {
   } else if (t.isCallExpression(node)) {
     gatherNodeParts(node.callee, parts);
   } else if (t.isObjectExpression(node) || t.isObjectPattern(node)) {
-    for (let prop of (node.properties: Array)) {
+    for (const prop of (node.properties: Array)) {
       gatherNodeParts(prop.key || prop.argument, parts);
     }
   }
@@ -69,10 +69,10 @@ function gatherNodeParts(node: Object, parts: Array) {
 
 //
 
-let collectorVisitor = {
+const collectorVisitor = {
   For(path) {
-    for (let key of (t.FOR_INIT_KEYS: Array)) {
-      let declar = path.get(key);
+    for (const key of (t.FOR_INIT_KEYS: Array)) {
+      const declar = path.get(key);
       if (declar.isVar()) path.scope.getFunctionParent().registerBinding("var", declar);
     }
   },
@@ -96,7 +96,7 @@ let collectorVisitor = {
   },
 
   ForXStatement(path, state) {
-    let left = path.get("left");
+    const left = path.get("left");
     if (left.isPattern() || left.isIdentifier()) {
       state.constantViolations.push(left);
     }
@@ -105,18 +105,18 @@ let collectorVisitor = {
   ExportDeclaration: {
     exit(path) {
       const { node, scope } = path;
-      let declar = node.declaration;
+      const declar = node.declaration;
       if (t.isClassDeclaration(declar) || t.isFunctionDeclaration(declar)) {
-        let id = declar.id;
+        const id = declar.id;
         if (!id) return;
 
-        let binding = scope.getBinding(id.name);
+        const binding = scope.getBinding(id.name);
         if (binding) binding.reference(path);
       } else if (t.isVariableDeclaration(declar)) {
-        for (let decl of (declar.declarations: Array<Object>)) {
-          let ids = t.getBindingIdentifiers(decl);
-          for (let name in ids) {
-            let binding = scope.getBinding(name);
+        for (const decl of (declar.declarations: Array<Object>)) {
+          const ids = t.getBindingIdentifiers(decl);
+          for (const name in ids) {
+            const binding = scope.getBinding(name);
             if (binding) binding.reference(path);
           }
         }
@@ -150,16 +150,16 @@ let collectorVisitor = {
   },
 
   ClassDeclaration(path) {
-    let id = path.node.id;
+    const id = path.node.id;
     if (!id) return;
 
-    let name = id.name;
+    const name = id.name;
     path.scope.bindings[name] = path.scope.getBinding(name);
   },
 
   Block(path) {
-    let paths = path.get("body");
-    for (let bodyPath of (paths: Array)) {
+    const paths = path.get("body");
+    for (const bodyPath of (paths: Array)) {
       if (bodyPath.isFunctionDeclaration()) {
         path.scope.getBlockParent().registerDeclaration(bodyPath);
       }
@@ -181,7 +181,7 @@ export default class Scope {
       return parentScope;
     }
 
-    let cached = getCache(path, parentScope, this);
+    const cached = getCache(path, parentScope, this);
     if (cached) return cached;
 
     this.uid = uid++;
@@ -225,7 +225,7 @@ export default class Scope {
    */
 
   generateDeclaredUidIdentifier(name: string = "temp") {
-    let id = this.generateUidIdentifier(name);
+    const id = this.generateUidIdentifier(name);
     this.push({ id });
     return id;
   }
@@ -252,7 +252,7 @@ export default class Scope {
       i++;
     } while (this.hasLabel(uid) || this.hasBinding(uid) || this.hasGlobal(uid) || this.hasReference(uid));
 
-    let program = this.getProgramParent();
+    const program = this.getProgramParent();
     program.references[uid] = true;
     program.uids[uid] = true;
 
@@ -309,7 +309,7 @@ export default class Scope {
     }
 
     if (t.isIdentifier(node)) {
-      let binding = this.getBinding(node.name);
+      const binding = this.getBinding(node.name);
       if (binding) {
         return binding.constant;
       } else {
@@ -328,7 +328,7 @@ export default class Scope {
     if (this.isStatic(node)) {
       return null;
     } else {
-      let id = this.generateUidIdentifierBasedOnNode(node);
+      const id = this.generateUidIdentifierBasedOnNode(node);
       if (!dontPush) this.push({ id });
       return id;
     }
@@ -355,7 +355,7 @@ export default class Scope {
   }
 
   rename(oldName: string, newName: string, block?) {
-    let binding = this.getBinding(oldName);
+    const binding = this.getBinding(oldName);
     if (binding) {
       newName = newName || this.generateUidIdentifier(oldName).name;
       return new Renamer(binding, oldName, newName).rename(block);
@@ -370,13 +370,13 @@ export default class Scope {
   }
 
   dump() {
-    let sep = repeat("-", 60);
+    const sep = repeat("-", 60);
     console.log(sep);
     let scope = this;
     do {
       console.log("#", scope.block.type);
-      for (let name in scope.bindings) {
-        let binding = scope.bindings[name];
+      for (const name in scope.bindings) {
+        const binding = scope.bindings[name];
         console.log(" -", name, {
           constant: binding.constant,
           references: binding.references,
@@ -389,10 +389,10 @@ export default class Scope {
   }
 
   toArray(node: Object, i?: number) {
-    let file = this.hub.file;
+    const file = this.hub.file;
 
     if (t.isIdentifier(node)) {
-      let binding = this.getBinding(node.name);
+      const binding = this.getBinding(node.name);
       if (binding && binding.constant && binding.path.isGenericType("Array")) return node;
     }
 
@@ -417,7 +417,7 @@ export default class Scope {
     }
 
     let helperName = "toArray";
-    let args = [node];
+    const args = [node];
     if (i === true) {
       helperName = "toConsumableArray";
     } else if (i) {
@@ -446,19 +446,19 @@ export default class Scope {
     } else if (path.isFunctionDeclaration()) {
       this.registerBinding("hoisted", path.get("id"), path);
     } else if (path.isVariableDeclaration()) {
-      let declarations = path.get("declarations");
-      for (let declar of (declarations: Array)) {
+      const declarations = path.get("declarations");
+      for (const declar of (declarations: Array)) {
         this.registerBinding(path.node.kind, declar);
       }
     } else if (path.isClassDeclaration()) {
       this.registerBinding("let", path);
     } else if (path.isImportDeclaration()) {
-      let specifiers = path.get("specifiers");
-      for (let specifier of (specifiers: Array)) {
+      const specifiers = path.get("specifiers");
+      for (const specifier of (specifiers: Array)) {
         this.registerBinding("module", specifier);
       }
     } else if (path.isExportDeclaration()) {
-      let declar = path.get("declaration");
+      const declar = path.get("declaration");
       if (declar.isClassDeclaration() || declar.isFunctionDeclaration() || declar.isVariableDeclaration()) {
         this.registerDeclaration(declar);
       }
@@ -476,9 +476,9 @@ export default class Scope {
   }
 
   registerConstantViolation(path: NodePath) {
-    let ids = path.getBindingIdentifiers();
-    for (let name in ids) {
-      let binding = this.getBinding(name);
+    const ids = path.getBindingIdentifiers();
+    for (const name in ids) {
+      const binding = this.getBinding(name);
       if (binding) binding.reassign(path);
     }
   }
@@ -487,18 +487,18 @@ export default class Scope {
     if (!kind) throw new ReferenceError("no `kind`");
 
     if (path.isVariableDeclaration()) {
-      let declarators: Array<NodePath> = path.get("declarations");
-      for (let declar of declarators) {
+      const declarators: Array<NodePath> = path.get("declarations");
+      for (const declar of declarators) {
         this.registerBinding(kind, declar);
       }
       return;
     }
 
-    let parent = this.getProgramParent();
-    let ids = path.getBindingIdentifiers(true);
+    const parent = this.getProgramParent();
+    const ids = path.getBindingIdentifiers(true);
 
-    for (let name in ids) {
-      for (let id of (ids[name]: Array<Object>)) {
+    for (const name in ids) {
+      for (const id of (ids[name]: Array<Object>)) {
         let local = this.getOwnBinding(name);
         if (local) {
           // same identifier so continue safely as we're likely trying to register it
@@ -562,7 +562,7 @@ export default class Scope {
 
   isPure(node, constantsOnly?: boolean) {
     if (t.isIdentifier(node)) {
-      let binding = this.getBinding(node.name);
+      const binding = this.getBinding(node.name);
       if (!binding) return false;
       if (constantsOnly) return binding.constant;
       return true;
@@ -570,19 +570,19 @@ export default class Scope {
       if (node.superClass && !this.isPure(node.superClass, constantsOnly)) return false;
       return this.isPure(node.body, constantsOnly);
     } else if (t.isClassBody(node)) {
-      for (let method of node.body) {
+      for (const method of node.body) {
         if (!this.isPure(method, constantsOnly)) return false;
       }
       return true;
     } else if (t.isBinary(node)) {
       return this.isPure(node.left, constantsOnly) && this.isPure(node.right, constantsOnly);
     } else if (t.isArrayExpression(node)) {
-      for (let elem of (node.elements: Array<Object>)) {
+      for (const elem of (node.elements: Array<Object>)) {
         if (!this.isPure(elem, constantsOnly)) return false;
       }
       return true;
     } else if (t.isObjectExpression(node)) {
-      for (let prop of (node.properties: Array<Object>)) {
+      for (const prop of (node.properties: Array<Object>)) {
         if (!this.isPure(prop, constantsOnly)) return false;
       }
       return true;
@@ -615,7 +615,7 @@ export default class Scope {
   getData(key) {
     let scope = this;
     do {
-      let data = scope.data[key];
+      const data = scope.data[key];
       if (data != null) return data;
     } while (scope = scope.parent);
   }
@@ -628,7 +628,7 @@ export default class Scope {
   removeData(key) {
     let scope = this;
     do {
-      let data = scope.data[key];
+      const data = scope.data[key];
       if (data != null) scope.data[key] = null;
     } while (scope = scope.parent);
   }
@@ -644,7 +644,7 @@ export default class Scope {
   }
 
   _crawl() {
-    let path = this.path;
+    const path = this.path;
 
     //
 
@@ -657,8 +657,8 @@ export default class Scope {
     // ForStatement - left, init
 
     if (path.isLoop()) {
-      for (let key of (t.FOR_INIT_KEYS: Array<string>)) {
-        let node = path.get(key);
+      for (const key of (t.FOR_INIT_KEYS: Array<string>)) {
+        const node = path.get(key);
         if (node.isBlockScoped()) this.registerBinding(node.node.kind, node);
       }
     }
@@ -682,8 +682,8 @@ export default class Scope {
     // Function - params, rest
 
     if (path.isFunction()) {
-      let params: Array<NodePath> = path.get("params");
-      for (let param of params) {
+      const params: Array<NodePath> = path.get("params");
+      for (const param of params) {
         this.registerBinding("param", param);
       }
     }
@@ -696,10 +696,10 @@ export default class Scope {
 
     // Program
 
-    let parent = this.getProgramParent();
+    const parent = this.getProgramParent();
     if (parent.crawling) return;
 
-    let state = {
+    const state = {
       references: [],
       constantViolations: [],
       assignments: [],
@@ -710,11 +710,11 @@ export default class Scope {
     this.crawling = false;
 
     // register assignments
-    for (let path of state.assignments) {
+    for (const path of state.assignments) {
       // register undeclared bindings as globals
-      let ids = path.getBindingIdentifiers();
+      const ids = path.getBindingIdentifiers();
       let programParent;
-      for (let name in ids) {
+      for (const name in ids) {
         if (path.scope.getBinding(name)) continue;
 
         programParent = programParent ||  path.scope.getProgramParent();
@@ -726,8 +726,8 @@ export default class Scope {
     }
 
     // register references
-    for (let ref of state.references) {
-      let binding = ref.scope.getBinding(ref.node.name);
+    for (const ref of state.references) {
+      const binding = ref.scope.getBinding(ref.node.name);
       if (binding) {
         binding.reference(ref);
       } else {
@@ -736,7 +736,7 @@ export default class Scope {
     }
 
     // register constant violations
-    for (let path of state.constantViolations) {
+    for (const path of state.constantViolations) {
       path.scope.registerConstantViolation(path);
     }
   }
@@ -763,15 +763,15 @@ export default class Scope {
       path = path.get("body");
     }
 
-    let unique = opts.unique;
-    let kind   = opts.kind || "var";
-    let blockHoist = opts._blockHoist == null ? 2 : opts._blockHoist;
+    const unique = opts.unique;
+    const kind   = opts.kind || "var";
+    const blockHoist = opts._blockHoist == null ? 2 : opts._blockHoist;
 
-    let dataKey = `declaration:${kind}:${blockHoist}`;
+    const dataKey = `declaration:${kind}:${blockHoist}`;
     let declarPath  = !unique && path.getData(dataKey);
 
     if (!declarPath) {
-      let declar = t.variableDeclaration(kind, []);
+      const declar = t.variableDeclaration(kind, []);
       declar._generated = true;
       declar._blockHoist = blockHoist;
 
@@ -779,7 +779,7 @@ export default class Scope {
       if (!unique) path.setData(dataKey, declarPath);
     }
 
-    let declarator = t.variableDeclarator(opts.id, opts.init);
+    const declarator = t.variableDeclarator(opts.id, opts.init);
     declarPath.node.declarations.push(declarator);
     this.registerBinding(kind, declarPath.get("declarations").pop());
   }
@@ -833,7 +833,7 @@ export default class Scope {
    */
 
   getAllBindings(): Object {
-    let ids = Object.create(null);
+    const ids = Object.create(null);
 
     let scope = this;
     do {
@@ -849,13 +849,13 @@ export default class Scope {
    */
 
   getAllBindingsOfKind(): Object {
-    let ids = Object.create(null);
+    const ids = Object.create(null);
 
-    for (let kind of (arguments: Array)) {
+    for (const kind of (arguments: Array)) {
       let scope = this;
       do {
-        for (let name in scope.bindings) {
-          let binding = scope.bindings[name];
+        for (const name in scope.bindings) {
+          const binding = scope.bindings[name];
           if (binding.kind === kind) ids[name] = binding;
         }
         scope = scope.parent;
@@ -884,7 +884,7 @@ export default class Scope {
     let scope = this;
 
     do {
-      let binding = scope.getOwnBinding(name);
+      const binding = scope.getOwnBinding(name);
       if (binding) return this.warnOnFlowBinding(binding);
     } while (scope = scope.parent);
   }
@@ -894,12 +894,12 @@ export default class Scope {
   }
 
   getBindingIdentifier(name: string) {
-    let info = this.getBinding(name);
+    const info = this.getBinding(name);
     return info && info.identifier;
   }
 
   getOwnBindingIdentifier(name: string) {
-    let binding = this.bindings[name];
+    const binding = this.bindings[name];
     return binding && binding.identifier;
   }
 
@@ -926,7 +926,7 @@ export default class Scope {
    */
 
   moveBindingTo(name, scope) {
-    let info = this.getBinding(name);
+    const info = this.getBinding(name);
     if (info) {
       info.scope.removeOwnBinding(name);
       info.scope = scope;
@@ -940,7 +940,7 @@ export default class Scope {
 
   removeBinding(name: string) {
     // clear literal binding
-    let info = this.getBinding(name);
+    const info = this.getBinding(name);
     if (info) {
       info.scope.removeOwnBinding(name);
     }
diff --git a/packages/babel-traverse/src/scope/lib/renamer.js b/packages/babel-traverse/src/scope/lib/renamer.js
index b8c1fb3b9c3f..1f05cb221a9c 100644
--- a/packages/babel-traverse/src/scope/lib/renamer.js
+++ b/packages/babel-traverse/src/scope/lib/renamer.js
@@ -1,7 +1,7 @@
 import Binding from "../binding";
 import * as t from "babel-types";
 
-let renameVisitor = {
+const renameVisitor = {
   ReferencedIdentifier({ node }, state) {
     if (node.name === state.oldName) {
       node.name = state.newName;
@@ -15,9 +15,9 @@ let renameVisitor = {
   },
 
   "AssignmentExpression|Declaration"(path, state) {
-    let ids = path.getOuterBindingIdentifiers();
+    const ids = path.getOuterBindingIdentifiers();
 
-    for (let name in ids) {
+    for (const name in ids) {
       if (name === state.oldName) ids[name].name = state.newName;
     }
   }
@@ -35,11 +35,11 @@ export default class Renamer {
   binding: Binding;
 
   maybeConvertFromExportDeclaration(parentDeclar) {
-    let exportDeclar = parentDeclar.parentPath.isExportDeclaration() && parentDeclar.parentPath;
+    const exportDeclar = parentDeclar.parentPath.isExportDeclaration() && parentDeclar.parentPath;
     if (!exportDeclar) return;
 
     // build specifiers that point back to this export declaration
-    let isDefault = exportDeclar.isExportDefaultDeclaration();
+    const isDefault = exportDeclar.isExportDefaultDeclaration();
 
     if (isDefault && (parentDeclar.isFunctionDeclaration() ||
         parentDeclar.isClassDeclaration()) && !parentDeclar.node.id) {
@@ -48,17 +48,17 @@ export default class Renamer {
       parentDeclar.node.id = parentDeclar.scope.generateUidIdentifier("default");
     }
 
-    let bindingIdentifiers = parentDeclar.getOuterBindingIdentifiers();
-    let specifiers = [];
+    const bindingIdentifiers = parentDeclar.getOuterBindingIdentifiers();
+    const specifiers = [];
 
-    for (let name in bindingIdentifiers) {
-      let localName = name === this.oldName ? this.newName : name;
-      let exportedName = isDefault ? "default" : name;
+    for (const name in bindingIdentifiers) {
+      const localName = name === this.oldName ? this.newName : name;
+      const exportedName = isDefault ? "default" : name;
       specifiers.push(t.exportSpecifier(t.identifier(localName), t.identifier(exportedName)));
     }
 
     if (specifiers.length) {
-      let aliasDeclar = t.exportNamedDeclaration(null, specifiers);
+      const aliasDeclar = t.exportNamedDeclaration(null, specifiers);
 
       // hoist to the top if it's a function
       if (parentDeclar.isFunctionDeclaration()) {
@@ -104,10 +104,10 @@ export default class Renamer {
   }
 
   rename(block?) {
-    let { binding, oldName, newName } = this;
-    let { scope, path } = binding;
+    const { binding, oldName, newName } = this;
+    const { scope, path } = binding;
 
-    let parentDeclar = path.find((path) => path.isDeclaration() || path.isFunctionExpression());
+    const parentDeclar = path.find((path) => path.isDeclaration() || path.isFunctionExpression());
     if (parentDeclar) {
       this.maybeConvertFromExportDeclaration(parentDeclar);
     }
diff --git a/packages/babel-traverse/src/visitors.js b/packages/babel-traverse/src/visitors.js
index 023cdb186c2d..f8de5b791de3 100644
--- a/packages/babel-traverse/src/visitors.js
+++ b/packages/babel-traverse/src/visitors.js
@@ -24,16 +24,16 @@ export function explode(visitor) {
   visitor._exploded = true;
 
   // normalise pipes
-  for (let nodeType in visitor) {
+  for (const nodeType in visitor) {
     if (shouldIgnoreKey(nodeType)) continue;
 
-    let parts: Array<string> = nodeType.split("|");
+    const parts: Array<string> = nodeType.split("|");
     if (parts.length === 1) continue;
 
-    let fns = visitor[nodeType];
+    const fns = visitor[nodeType];
     delete visitor[nodeType];
 
-    for (let part of parts) {
+    for (const part of parts) {
       visitor[part] = fns;
     }
   }
@@ -52,15 +52,15 @@ export function explode(visitor) {
   ensureCallbackArrays(visitor);
 
   // add type wrappers
-  for (let nodeType of (Object.keys(visitor): Array)) {
+  for (const nodeType of (Object.keys(visitor): Array)) {
     if (shouldIgnoreKey(nodeType)) continue;
 
-    let wrapper = virtualTypes[nodeType];
+    const wrapper = virtualTypes[nodeType];
     if (!wrapper) continue;
 
     // wrap all the functions
-    let fns = visitor[nodeType];
-    for (let type in fns) {
+    const fns = visitor[nodeType];
+    for (const type in fns) {
       fns[type] = wrapCheck(wrapper, fns[type]);
     }
 
@@ -68,7 +68,7 @@ export function explode(visitor) {
     delete visitor[nodeType];
 
     if (wrapper.types) {
-      for (let type of (wrapper.types: Array<string>)) {
+      for (const type of (wrapper.types: Array<string>)) {
         // merge the visitor if necessary or just put it back in
         if (visitor[type]) {
           mergePair(visitor[type], fns);
@@ -82,14 +82,14 @@ export function explode(visitor) {
   }
 
   // add aliases
-  for (let nodeType in visitor) {
+  for (const nodeType in visitor) {
     if (shouldIgnoreKey(nodeType)) continue;
 
-    let fns = visitor[nodeType];
+    const fns = visitor[nodeType];
 
     let aliases: ?Array<string> = t.FLIPPED_ALIAS_KEYS[nodeType];
 
-    let deprecratedKey = t.DEPRECATED_KEYS[nodeType];
+    const deprecratedKey = t.DEPRECATED_KEYS[nodeType];
     if (deprecratedKey) {
       console.trace(`Visitor defined for ${nodeType} but it has been renamed to ${deprecratedKey}`);
       aliases = [deprecratedKey];
@@ -100,8 +100,8 @@ export function explode(visitor) {
     // clear it from the visitor
     delete visitor[nodeType];
 
-    for (let alias of aliases) {
-      let existing = visitor[alias];
+    for (const alias of aliases) {
+      const existing = visitor[alias];
       if (existing) {
         mergePair(existing, fns);
       } else {
@@ -110,7 +110,7 @@ export function explode(visitor) {
     }
   }
 
-  for (let nodeType in visitor) {
+  for (const nodeType in visitor) {
     if (shouldIgnoreKey(nodeType)) continue;
 
     ensureCallbackArrays(visitor[nodeType]);
@@ -126,7 +126,7 @@ export function verify(visitor) {
     throw new Error(messages.get("traverseVerifyRootFunction"));
   }
 
-  for (let nodeType in visitor) {
+  for (const nodeType in visitor) {
     if (nodeType === "enter" || nodeType === "exit") {
       validateVisitorMethods(nodeType, visitor[nodeType]);
     }
@@ -137,9 +137,9 @@ export function verify(visitor) {
       throw new Error(messages.get("traverseVerifyNodeType", nodeType));
     }
 
-    let visitors = visitor[nodeType];
+    const visitors = visitor[nodeType];
     if (typeof visitors === "object") {
-      for (let visitorKey in visitors) {
+      for (const visitorKey in visitors) {
         if (visitorKey === "enter" || visitorKey === "exit") {
           // verify that it just contains functions
           validateVisitorMethods(`${nodeType}.${visitorKey}`, visitors[visitorKey]);
@@ -154,8 +154,8 @@ export function verify(visitor) {
 }
 
 function validateVisitorMethods(path, val) {
-  let fns = [].concat(val);
-  for (let fn of fns) {
+  const fns = [].concat(val);
+  for (const fn of fns) {
     if (typeof fn !== "function") {
       throw new TypeError(`Non-function found defined in ${path} with type ${typeof fn}`);
     }
@@ -163,15 +163,15 @@ function validateVisitorMethods(path, val) {
 }
 
 export function merge(visitors: Array, states: Array = [], wrapper?: ?Function) {
-  let rootVisitor = {};
+  const rootVisitor = {};
 
   for (let i = 0; i < visitors.length; i++) {
-    let visitor = visitors[i];
-    let state = states[i];
+    const visitor = visitors[i];
+    const state = states[i];
 
     explode(visitor);
 
-    for (let type in visitor) {
+    for (const type in visitor) {
       let visitorType = visitor[type];
 
       // if we have state or wrapper then overload the callbacks to take it
@@ -179,7 +179,7 @@ export function merge(visitors: Array, states: Array = [], wrapper?: ?Function)
         visitorType = wrapWithStateOrWrapper(visitorType, state, wrapper);
       }
 
-      let nodeVisitor = rootVisitor[type] = rootVisitor[type] || {};
+      const nodeVisitor = rootVisitor[type] = rootVisitor[type] || {};
       mergePair(nodeVisitor, visitorType);
     }
   }
@@ -188,9 +188,9 @@ export function merge(visitors: Array, states: Array = [], wrapper?: ?Function)
 }
 
 function wrapWithStateOrWrapper(oldVisitor, state, wrapper: ?Function) {
-  let newVisitor = {};
+  const newVisitor = {};
 
-  for (let key in oldVisitor) {
+  for (const key in oldVisitor) {
     let fns = oldVisitor[key];
 
     // not an enter/exit array of callbacks
@@ -219,10 +219,10 @@ function wrapWithStateOrWrapper(oldVisitor, state, wrapper: ?Function) {
 }
 
 function ensureEntranceObjects(obj) {
-  for (let key in obj) {
+  for (const key in obj) {
     if (shouldIgnoreKey(key)) continue;
 
-    let fns = obj[key];
+    const fns = obj[key];
     if (typeof fns === "function") {
       obj[key] = { enter: fns };
     }
@@ -235,7 +235,7 @@ function ensureCallbackArrays(obj) {
 }
 
 function wrapCheck(wrapper, fn) {
-  let newFn = function (path) {
+  const newFn = function (path) {
     if (wrapper.checkPath(path)) {
       return fn.apply(this, arguments);
     }
@@ -258,7 +258,7 @@ function shouldIgnoreKey(key) {
 }
 
 function mergePair(dest, src) {
-  for (let key in src) {
+  for (const key in src) {
     dest[key] = [].concat(dest[key] || [], src[key]);
   }
 }
diff --git a/packages/babel-traverse/test/ancestry.js b/packages/babel-traverse/test/ancestry.js
index 43e09859ef66..8d8f01394829 100644
--- a/packages/babel-traverse/test/ancestry.js
+++ b/packages/babel-traverse/test/ancestry.js
@@ -1,63 +1,63 @@
-let traverse = require("../lib").default;
-let assert = require("assert");
-let parse = require("babylon").parse;
+const traverse = require("../lib").default;
+const assert = require("assert");
+const parse = require("babylon").parse;
 
 describe("path/ancestry", function () {
   describe("isAncestor", function () {
-    let ast = parse("var a = 1; 'a';");
+    const ast = parse("var a = 1; 'a';");
 
     it("returns true if ancestor", function() {
-      let paths = [];
+      const paths = [];
       traverse(ast, {
         "Program|NumericLiteral"(path) {
           paths.push(path);
         },
       });
 
-      let [ programPath, numberPath ] = paths;
+      const [ programPath, numberPath ] = paths;
 
       assert(programPath.isAncestor(numberPath));
     });
 
     it("returns false if not ancestor", function() {
-      let paths = [];
+      const paths = [];
       traverse(ast, {
         "Program|NumericLiteral|StringLiteral"(path) {
           paths.push(path);
         }
       });
 
-      let [ , numberPath, stringPath ] = paths;
+      const [ , numberPath, stringPath ] = paths;
 
       assert(!stringPath.isAncestor(numberPath));
     });
   });
 
   describe("isDescendant", function () {
-    let ast = parse("var a = 1; 'a';");
+    const ast = parse("var a = 1; 'a';");
 
     it("returns true if descendant", function() {
-      let paths = [];
+      const paths = [];
       traverse(ast, {
         "Program|NumericLiteral"(path) {
           paths.push(path);
         },
       });
 
-      let [ programPath, numberPath ] = paths;
+      const [ programPath, numberPath ] = paths;
 
       assert(numberPath.isDescendant(programPath));
     });
 
     it("returns false if not descendant", function() {
-      let paths = [];
+      const paths = [];
       traverse(ast, {
         "Program|NumericLiteral|StringLiteral"(path) {
           paths.push(path);
         }
       });
 
-      let [ , numberPath, stringPath ] = paths;
+      const [ , numberPath, stringPath ] = paths;
 
       assert(!numberPath.isDescendant(stringPath));
     });
diff --git a/packages/babel-traverse/test/evaluation.js b/packages/babel-traverse/test/evaluation.js
index 956d656eb3d3..3c53fbf6158e 100644
--- a/packages/babel-traverse/test/evaluation.js
+++ b/packages/babel-traverse/test/evaluation.js
@@ -1,9 +1,9 @@
-let traverse = require("../lib").default;
-let assert = require("assert");
-let parse = require("babylon").parse;
+const traverse = require("../lib").default;
+const assert = require("assert");
+const parse = require("babylon").parse;
 
 function getPath(code) {
-  let ast = parse(code);
+  const ast = parse(code);
   let path;
   traverse(ast, {
     Program: function (_path) {
diff --git a/packages/babel-traverse/test/family.js b/packages/babel-traverse/test/family.js
index c6fc297f86a6..6e9a497df746 100644
--- a/packages/babel-traverse/test/family.js
+++ b/packages/babel-traverse/test/family.js
@@ -1,10 +1,10 @@
-let traverse = require("../lib").default;
-let assert = require("assert");
-let parse = require("babylon").parse;
+const traverse = require("../lib").default;
+const assert = require("assert");
+const parse = require("babylon").parse;
 
 describe("path/family", function () {
   describe("getBindingIdentifiers", function () {
-    let ast = parse("var a = 1, {b} = c, [d] = e; function f() {}");
+    const ast = parse("var a = 1, {b} = c, [d] = e; function f() {}");
     let nodes = {}, paths = {}, outerNodes = {}, outerPaths = {};
     traverse(ast, {
       VariableDeclaration(path) {
diff --git a/packages/babel-traverse/test/scope.js b/packages/babel-traverse/test/scope.js
index 332a3eabd9b9..db51118fcfe8 100644
--- a/packages/babel-traverse/test/scope.js
+++ b/packages/babel-traverse/test/scope.js
@@ -1,9 +1,9 @@
-let traverse = require("../lib").default;
-let assert = require("assert");
-let parse = require("babylon").parse;
+const traverse = require("../lib").default;
+const assert = require("assert");
+const parse = require("babylon").parse;
 
 function getPath(code) {
-  let ast = parse(code);
+  const ast = parse(code);
   let path;
   traverse(ast, {
     Program: function (_path) {
diff --git a/packages/babel-traverse/test/traverse.js b/packages/babel-traverse/test/traverse.js
index cee715715059..ce87f9973ffa 100644
--- a/packages/babel-traverse/test/traverse.js
+++ b/packages/babel-traverse/test/traverse.js
@@ -1,23 +1,23 @@
-let traverse = require("../lib").default;
-let assert   = require("assert");
-let _        = require("lodash");
-let parse = require("babylon").parse;
+const traverse = require("../lib").default;
+const assert   = require("assert");
+const _        = require("lodash");
+const parse = require("babylon").parse;
 
 describe("traverse", function () {
-  let code = `
+  const code = `
     var foo = "bar";
     this.test = "wow";
   `;
-  let ast = parse(code);
-  let program = ast.program;
-  let body = program.body;
+  const ast = parse(code);
+  const program = ast.program;
+  const body = program.body;
 
   it("traverse replace", function () {
-    let replacement = {
+    const replacement = {
       type: "StringLiteral",
       value: "foo"
     };
-    let ast2 = _.cloneDeep(program);
+    const ast2 = _.cloneDeep(program);
 
     traverse(ast2, {
       enter: function (path) {
@@ -29,12 +29,12 @@ describe("traverse", function () {
   });
 
   it("traverse", function () {
-    let expect = [
+    const expect = [
       body[0], body[0].declarations[0], body[0].declarations[0].id, body[0].declarations[0].init,
       body[1], body[1].expression, body[1].expression.left, body[1].expression.left.object, body[1].expression.left.property, body[1].expression.right
     ];
 
-    let actual = [];
+    const actual = [];
 
     traverse(program, {
       enter: function (path) {
@@ -54,12 +54,12 @@ describe("traverse", function () {
   });
 
   it("traverse blacklistTypes", function () {
-    let expect = [
+    const expect = [
       body[0], body[0].declarations[0], body[0].declarations[0].id, body[0].declarations[0].init,
       body[1], body[1].expression, body[1].expression.right
     ];
 
-    let actual = [];
+    const actual = [];
 
     traverse(program, {
       blacklist: ["MemberExpression"],
@@ -85,8 +85,8 @@ describe("traverse", function () {
   });
 
   it("clearCache", function () {
-    let paths = [];
-    let scopes = [];
+    const paths = [];
+    const scopes = [];
     traverse(ast, {
       enter(path) {
         scopes.push(path.scope);
@@ -97,8 +97,8 @@ describe("traverse", function () {
 
     traverse.clearCache();
 
-    let paths2 = [];
-    let scopes2 = [];
+    const paths2 = [];
+    const scopes2 = [];
     traverse(ast, {
       enter(path) {
         scopes2.push(path.scope);
@@ -114,7 +114,7 @@ describe("traverse", function () {
   });
 
   it("clearPath", function () {
-    let paths = [];
+    const paths = [];
     traverse(ast, {
       enter(path) {
         paths.push(path);
@@ -123,7 +123,7 @@ describe("traverse", function () {
 
     traverse.clearCache.clearPath();
 
-    let paths2 = [];
+    const paths2 = [];
     traverse(ast, {
       enter(path) {
         paths2.push(path);
@@ -136,7 +136,7 @@ describe("traverse", function () {
   });
 
   it("clearScope", function () {
-    let scopes = [];
+    const scopes = [];
     traverse(ast, {
       enter(path) {
         scopes.push(path.scope);
@@ -146,7 +146,7 @@ describe("traverse", function () {
 
     traverse.clearCache.clearScope();
 
-    let scopes2 = [];
+    const scopes2 = [];
     traverse(ast, {
       enter(path) {
         scopes2.push(path.scope);
diff --git a/packages/babel-types/src/converters.js b/packages/babel-types/src/converters.js
index dcfc14d9f999..f350230ddf6d 100644
--- a/packages/babel-types/src/converters.js
+++ b/packages/babel-types/src/converters.js
@@ -24,10 +24,10 @@ export function toComputedKey(node: Object, key: Object = node.key || node.prope
 export function toSequenceExpression(nodes: Array<Object>, scope: Scope): ?Object {
   if (!nodes || !nodes.length) return;
 
-  let declars = [];
+  const declars = [];
   let bailed  = false;
 
-  let result = convert(nodes);
+  const result = convert(nodes);
   if (bailed) return;
 
   for (let i = 0; i < declars.length; i++) {
@@ -38,9 +38,9 @@ export function toSequenceExpression(nodes: Array<Object>, scope: Scope): ?Objec
 
   function convert(nodes) {
     let ensureLastUndefined = false;
-    let exprs   = [];
+    const exprs   = [];
 
-    for (let node of (nodes: Array)) {
+    for (const node of (nodes: Array)) {
       if (t.isExpression(node)) {
         exprs.push(node);
       } else if (t.isExpressionStatement(node)) {
@@ -48,9 +48,9 @@ export function toSequenceExpression(nodes: Array<Object>, scope: Scope): ?Objec
       } else if (t.isVariableDeclaration(node)) {
         if (node.kind !== "var") return bailed = true; // bailed
 
-        for (let declar of (node.declarations: Array)) {
-          let bindings = t.getBindingIdentifiers(declar);
-          for (let key in bindings) {
+        for (const declar of (node.declarations: Array)) {
+          const bindings = t.getBindingIdentifiers(declar);
+          for (const key in bindings) {
             declars.push({
               kind: node.kind,
               id: bindings[key]
@@ -65,8 +65,8 @@ export function toSequenceExpression(nodes: Array<Object>, scope: Scope): ?Objec
         ensureLastUndefined = true;
         continue;
       } else if (t.isIfStatement(node)) {
-        let consequent = node.consequent ? convert([node.consequent]) : scope.buildUndefinedNode();
-        let alternate = node.alternate ? convert([node.alternate]) : scope.buildUndefinedNode();
+        const consequent = node.consequent ? convert([node.consequent]) : scope.buildUndefinedNode();
+        const alternate = node.alternate ? convert([node.alternate]) : scope.buildUndefinedNode();
         if (!consequent || !alternate) return bailed = true;
 
         exprs.push(t.conditionalExpression(node.test, consequent, alternate));
@@ -283,8 +283,8 @@ export function valueToNode(value: any): Object {
 
   // regexes
   if (isRegExp(value)) {
-    let pattern = value.source;
-    let flags = value.toString().match(/\/([a-z]+|)$/)[1];
+    const pattern = value.source;
+    const flags = value.toString().match(/\/([a-z]+|)$/)[1];
     return t.regExpLiteral(pattern, flags);
   }
 
@@ -295,8 +295,8 @@ export function valueToNode(value: any): Object {
 
   // object
   if (isPlainObject(value)) {
-    let props = [];
-    for (let key in value) {
+    const props = [];
+    for (const key in value) {
       let nodeKey;
       if (t.isValidIdentifier(key)) {
         nodeKey = t.identifier(key);
diff --git a/packages/babel-types/src/definitions/core.js b/packages/babel-types/src/definitions/core.js
index 19a37158a7e2..276db2048a17 100644
--- a/packages/babel-types/src/definitions/core.js
+++ b/packages/babel-types/src/definitions/core.js
@@ -425,7 +425,7 @@ defineType("MemberExpression", {
     },
     property: {
       validate(node, key, val) {
-        let expectedType = node.computed ? "Expression" : "Identifier";
+        const expectedType = node.computed ? "Expression" : "Identifier";
         assertNodeType(expectedType)(node, key, val);
       }
     },
@@ -489,7 +489,7 @@ defineType("ObjectMethod", {
     },
     key: {
       validate(node, key, val) {
-        let expectedTypes = node.computed ? ["Expression"] : ["Identifier", "StringLiteral", "NumericLiteral"];
+        const expectedTypes = node.computed ? ["Expression"] : ["Identifier", "StringLiteral", "NumericLiteral"];
         assertNodeType(...expectedTypes)(node, key, val);
       }
     },
@@ -521,7 +521,7 @@ defineType("ObjectProperty", {
     },
     key: {
       validate(node, key, val) {
-        let expectedTypes = node.computed ? ["Expression"] : ["Identifier", "StringLiteral", "NumericLiteral"];
+        const expectedTypes = node.computed ? ["Expression"] : ["Identifier", "StringLiteral", "NumericLiteral"];
         assertNodeType(...expectedTypes)(node, key, val);
       }
     },
diff --git a/packages/babel-types/src/definitions/es2015.js b/packages/babel-types/src/definitions/es2015.js
index 4f4ff0807ce7..47c029857793 100644
--- a/packages/babel-types/src/definitions/es2015.js
+++ b/packages/babel-types/src/definitions/es2015.js
@@ -264,7 +264,7 @@ defineType("ClassMethod", {
     },
     key: {
       validate(node, key, val) {
-        let expectedTypes = node.computed ? ["Expression"] : ["Identifier", "StringLiteral", "NumericLiteral"];
+        const expectedTypes = node.computed ? ["Expression"] : ["Identifier", "StringLiteral", "NumericLiteral"];
         assertNodeType(...expectedTypes)(node, key, val);
       }
     },
diff --git a/packages/babel-types/src/definitions/index.js b/packages/babel-types/src/definitions/index.js
index d4ae802350d7..17e3ca24a686 100644
--- a/packages/babel-types/src/definitions/index.js
+++ b/packages/babel-types/src/definitions/index.js
@@ -1,10 +1,10 @@
 import * as t from "../index";
 
-export let VISITOR_KEYS = {};
-export let ALIAS_KEYS = {};
-export let NODE_FIELDS = {};
-export let BUILDER_KEYS = {};
-export let DEPRECATED_KEYS = {};
+export const VISITOR_KEYS = {};
+export const ALIAS_KEYS = {};
+export const NODE_FIELDS = {};
+export const BUILDER_KEYS = {};
+export const DEPRECATED_KEYS = {};
 
 function getType(val) {
   if (Array.isArray(val)) {
@@ -48,7 +48,7 @@ export function assertNodeType(...types: Array<string>): Function {
   function validate(node, key, val) {
     let valid = false;
 
-    for (let type of types) {
+    for (const type of types) {
       if (t.is(type, val)) {
         valid = true;
         break;
@@ -72,7 +72,7 @@ export function assertNodeOrValueType(...types: Array<string>): Function {
   function validate(node, key, val) {
     let valid = false;
 
-    for (let type of types) {
+    for (const type of types) {
       if (getType(val) === type || t.is(type, val)) {
         valid = true;
         break;
@@ -94,7 +94,7 @@ export function assertNodeOrValueType(...types: Array<string>): Function {
 
 export function assertValueType(type: string): Function {
   function validate(node, key, val) {
-    let valid = getType(val) === type;
+    const valid = getType(val) === type;
 
     if (!valid) {
       throw new TypeError(`Property ${key} expected type of ${type} but got ${getType(val)}`);
@@ -108,7 +108,7 @@ export function assertValueType(type: string): Function {
 
 export function chain(...fns: Array<Function>): Function {
   function validate(...args) {
-    for (let fn of fns) {
+    for (const fn of fns) {
       fn(...args);
     }
   }
@@ -127,7 +127,7 @@ export default function defineType(
     deprecatedAlias?: string;
   } = {},
 ) {
-  let inherits = (opts.inherits && store[opts.inherits]) || {};
+  const inherits = (opts.inherits && store[opts.inherits]) || {};
 
   opts.fields  = opts.fields || inherits.fields || {};
   opts.visitor = opts.visitor || inherits.visitor || [];
@@ -139,12 +139,12 @@ export default function defineType(
   }
 
   // ensure all field keys are represented in `fields`
-  for (let key of (opts.visitor.concat(opts.builder): Array<string>)) {
+  for (const key of (opts.visitor.concat(opts.builder): Array<string>)) {
     opts.fields[key] = opts.fields[key] || {};
   }
 
-  for (let key in opts.fields) {
-    let field = opts.fields[key];
+  for (const key in opts.fields) {
+    const field = opts.fields[key];
 
     if (opts.builder.indexOf(key) === -1) {
       field.optional = true;
@@ -164,4 +164,4 @@ export default function defineType(
   store[type] = opts;
 }
 
-let store = {};
+const store = {};
diff --git a/packages/babel-types/src/flow.js b/packages/babel-types/src/flow.js
index b2dbd43dbc53..996a11c8f4fa 100644
--- a/packages/babel-types/src/flow.js
+++ b/packages/babel-types/src/flow.js
@@ -6,7 +6,7 @@ import * as t from "./index";
  */
 
 export function createUnionTypeAnnotation(types: Array<Object>) {
-  let flattened  = removeTypeDuplicates(types);
+  const flattened  = removeTypeDuplicates(types);
 
   if (flattened.length === 1) {
     return flattened[0];
@@ -20,16 +20,16 @@ export function createUnionTypeAnnotation(types: Array<Object>) {
  */
 
 export function removeTypeDuplicates(nodes: Array<Object>): Array<Object> {
-  let generics = {};
-  let bases = {};
+  const generics = {};
+  const bases = {};
 
   // store union type groups to circular references
-  let typeGroups = [];
+  const typeGroups = [];
 
-  let types = [];
+  const types = [];
 
   for (let i = 0; i < nodes.length; i++) {
-    let node = nodes[i];
+    const node = nodes[i];
     if (!node) continue;
 
     // detect duplicates
@@ -59,7 +59,7 @@ export function removeTypeDuplicates(nodes: Array<Object>): Array<Object> {
 
     // find a matching generic type and merge and deduplicate the type parameters
     if (t.isGenericTypeAnnotation(node)) {
-      let name = node.id.name;
+      const name = node.id.name;
 
       if (generics[name]) {
         let existing = generics[name];
@@ -83,12 +83,12 @@ export function removeTypeDuplicates(nodes: Array<Object>): Array<Object> {
   }
 
   // add back in bases
-  for (let type in bases) {
+  for (const type in bases) {
     types.push(bases[type]);
   }
 
   // add back in generics
-  for (let name in generics) {
+  for (const name in generics) {
     types.push(generics[name]);
   }
 
diff --git a/packages/babel-types/src/index.js b/packages/babel-types/src/index.js
index a5ae2c93257e..09cbe39ad1fc 100644
--- a/packages/babel-types/src/index.js
+++ b/packages/babel-types/src/index.js
@@ -4,7 +4,7 @@ import loClone from "lodash/clone";
 import each from "lodash/each";
 import uniq from "lodash/uniq";
 
-let t = exports;
+const t = exports;
 
 /**
  * Registers `is[Type]` and `assert[Type]` generated functions for a given `type`.
@@ -62,7 +62,7 @@ export { _react as react };
  * Registers `is[Type]` and `assert[Type]` for all types.
  */
 
-for (let type in t.VISITOR_KEYS) {
+for (const type in t.VISITOR_KEYS) {
   registerType(type);
 }
 
@@ -74,7 +74,7 @@ t.FLIPPED_ALIAS_KEYS = {};
 
 each(t.ALIAS_KEYS, function (aliases, type) {
   each(aliases, function (alias) {
-    let types = t.FLIPPED_ALIAS_KEYS[alias] = t.FLIPPED_ALIAS_KEYS[alias] || [];
+    const types = t.FLIPPED_ALIAS_KEYS[alias] = t.FLIPPED_ALIAS_KEYS[alias] || [];
     types.push(type);
   });
 });
@@ -102,7 +102,7 @@ export const TYPES = Object.keys(t.VISITOR_KEYS)
 export function is(type: string, node: Object, opts?: Object): boolean {
   if (!node) return false;
 
-  let matches = isType(node.type, type);
+  const matches = isType(node.type, type);
   if (!matches) return false;
 
   if (typeof opts === "undefined") {
@@ -123,11 +123,11 @@ export function isType(nodeType: string, targetType: string): boolean {
   // targetType was a primary node type, so there's no need to check the aliases.
   if (t.ALIAS_KEYS[targetType]) return false;
 
-  let aliases: ?Array<string> = t.FLIPPED_ALIAS_KEYS[targetType];
+  const aliases: ?Array<string> = t.FLIPPED_ALIAS_KEYS[targetType];
   if (aliases) {
     if (aliases[0] === nodeType) return true;
 
-    for (let alias of aliases) {
+    for (const alias of aliases) {
       if (nodeType === alias) return true;
     }
   }
@@ -148,13 +148,13 @@ each(t.BUILDER_KEYS, function (keys, type) {
       );
     }
 
-    let node = {};
+    const node = {};
     node.type = type;
 
     let i = 0;
 
-    for (let key of (keys: Array<string>)) {
-      let field = t.NODE_FIELDS[type][key];
+    for (const key of (keys: Array<string>)) {
+      const field = t.NODE_FIELDS[type][key];
 
       let arg = arguments[i++];
       if (arg === undefined) arg = loClone(field.default);
@@ -162,7 +162,7 @@ each(t.BUILDER_KEYS, function (keys, type) {
       node[key] = arg;
     }
 
-    for (let key in node) {
+    for (const key in node) {
       validate(node, key, node[key]);
     }
 
@@ -177,8 +177,8 @@ each(t.BUILDER_KEYS, function (keys, type) {
  * Description
  */
 
-for (let type in t.DEPRECATED_KEYS) {
-  let newType = t.DEPRECATED_KEYS[type];
+for (const type in t.DEPRECATED_KEYS) {
+  const newType = t.DEPRECATED_KEYS[type];
 
   function proxy(fn) {
     return function () {
@@ -199,10 +199,10 @@ for (let type in t.DEPRECATED_KEYS) {
 export function validate(node?: Object, key: string, val: any) {
   if (!node) return;
 
-  let fields = t.NODE_FIELDS[node.type];
+  const fields = t.NODE_FIELDS[node.type];
   if (!fields) return;
 
-  let field = fields[key];
+  const field = fields[key];
   if (!field || !field.validate) return;
   if (field.optional && val == null) return;
 
@@ -214,9 +214,9 @@ export function validate(node?: Object, key: string, val: any) {
  */
 
 export function shallowEqual(actual: Object, expected: Object): boolean {
-  let keys = Object.keys(expected);
+  const keys = Object.keys(expected);
 
-  for (let key of (keys: Array<string>)) {
+  for (const key of (keys: Array<string>)) {
     if (actual[key] !== expected[key]) {
       return false;
     }
@@ -260,8 +260,8 @@ export function ensureBlock(node: Object, key: string = "body"): Object {
 
 export function clone(node: Object): Object {
   if (!node) return node;
-  let newNode = {};
-  for (let key in node) {
+  const newNode = {};
+  for (const key in node) {
     if (key[0] === "_") continue;
     newNode[key] = node[key];
   }
@@ -273,7 +273,7 @@ export function clone(node: Object): Object {
  */
 
 export function cloneWithoutLoc(node: Object): Object {
-  let newNode = clone(node);
+  const newNode = clone(node);
   delete newNode.loc;
   return newNode;
 }
@@ -285,9 +285,9 @@ export function cloneWithoutLoc(node: Object): Object {
 
 export function cloneDeep(node: Object): Object {
   if (!node) return node;
-  let newNode = {};
+  const newNode = {};
 
-  for (let key in node) {
+  for (const key in node) {
     if (key[0] === "_") continue;
 
     let val = node[key];
@@ -315,17 +315,17 @@ export function cloneDeep(node: Object): Object {
  */
 
 export function buildMatchMemberExpression(match:string, allowPartial?: boolean): Function {
-  let parts = match.split(".");
+  const parts = match.split(".");
 
   return function (member) {
     // not a member expression
     if (!t.isMemberExpression(member)) return false;
 
-    let search = [member];
+    const search = [member];
     let i = 0;
 
     while (search.length) {
-      let node = search.shift();
+      const node = search.shift();
 
       if (allowPartial && i === parts.length) {
         return true;
@@ -366,7 +366,7 @@ export function buildMatchMemberExpression(match:string, allowPartial?: boolean)
  */
 
 export function removeComments(node: Object): Object {
-  for (let key of t.COMMENT_KEYS) {
+  for (const key of t.COMMENT_KEYS) {
     delete node[key];
   }
   return node;
@@ -409,19 +409,19 @@ export function inherits(child: Object, parent: Object): Object {
   if (!child || !parent) return child;
 
   // optionally inherit specific properties if not null
-  for (let key of (t.INHERIT_KEYS.optional: Array<string>)) {
+  for (const key of (t.INHERIT_KEYS.optional: Array<string>)) {
     if (child[key] == null) {
       child[key] = parent[key];
     }
   }
 
   // force inherit "private" properties
-  for (let key in parent) {
+  for (const key in parent) {
     if (key[0] === "_") child[key] = parent[key];
   }
 
   // force inherit select properties
-  for (let key of (t.INHERIT_KEYS.force: Array<string>)) {
+  for (const key of (t.INHERIT_KEYS.force: Array<string>)) {
     child[key] = parent[key];
   }
 
@@ -460,17 +460,17 @@ toFastProperties(t.VISITOR_KEYS);
 export function traverseFast(node: Node, enter: (node: Node) => void, opts?: Object) {
   if (!node) return;
 
-  let keys = t.VISITOR_KEYS[node.type];
+  const keys = t.VISITOR_KEYS[node.type];
   if (!keys) return;
 
   opts = opts || {};
   enter(node, opts);
 
-  for (let key of keys) {
-    let subNode = node[key];
+  for (const key of keys) {
+    const subNode = node[key];
 
     if (Array.isArray(subNode)) {
-      for (let node of subNode) {
+      for (const node of subNode) {
         traverseFast(node, enter, opts);
       }
     } else {
@@ -496,17 +496,17 @@ const CLEAR_KEYS_PLUS_COMMENTS: Array = t.COMMENT_KEYS.concat([
 
 export function removeProperties(node: Node, opts?: Object): void {
   opts = opts || {};
-  let map = opts.preserveComments ? CLEAR_KEYS : CLEAR_KEYS_PLUS_COMMENTS;
-  for (let key of map) {
+  const map = opts.preserveComments ? CLEAR_KEYS : CLEAR_KEYS_PLUS_COMMENTS;
+  for (const key of map) {
     if (node[key] != null) node[key] = undefined;
   }
 
-  for (let key in node) {
+  for (const key in node) {
     if (key[0] === "_" && node[key] != null) node[key] = undefined;
   }
 
-  let syms: Array<Symbol> = Object.getOwnPropertySymbols(node);
-  for (let sym of syms) {
+  const syms: Array<Symbol> = Object.getOwnPropertySymbols(node);
+  for (const sym of syms) {
     node[sym] = null;
   }
 }
diff --git a/packages/babel-types/src/react.js b/packages/babel-types/src/react.js
index 9b665d224d3e..ebf3829caee4 100644
--- a/packages/babel-types/src/react.js
+++ b/packages/babel-types/src/react.js
@@ -1,6 +1,6 @@
 import * as t from "./index";
 
-export let isReactComponent = t.buildMatchMemberExpression("React.Component");
+export const isReactComponent = t.buildMatchMemberExpression("React.Component");
 
 export function isCompatTag(tagName?: string): boolean {
   return !!tagName && /^[a-z]|\-/.test(tagName);
@@ -10,7 +10,7 @@ function cleanJSXElementLiteralChild(
   child: { value: string },
   args: Array<Object>,
 ) {
-  let lines = child.value.split(/\r\n|\n|\r/);
+  const lines = child.value.split(/\r\n|\n|\r/);
 
   let lastNonEmptyLine = 0;
 
@@ -23,11 +23,11 @@ function cleanJSXElementLiteralChild(
   let str = "";
 
   for (let i = 0; i < lines.length; i++) {
-    let line = lines[i];
+    const line = lines[i];
 
-    let isFirstLine = i === 0;
-    let isLastLine = i === lines.length - 1;
-    let isLastNonEmptyLine = i === lastNonEmptyLine;
+    const isFirstLine = i === 0;
+    const isLastLine = i === lines.length - 1;
+    const isLastNonEmptyLine = i === lastNonEmptyLine;
 
     // replace rendered whitespace tabs with spaces
     let trimmedLine = line.replace(/\t/g, " ");
@@ -55,7 +55,7 @@ function cleanJSXElementLiteralChild(
 }
 
 export function buildChildren(node: Object): Array<Object> {
-  let elems = [];
+  const elems = [];
 
   for (let i = 0; i < node.children.length; i++) {
     let child = node.children[i];
diff --git a/packages/babel-types/src/retrievers.js b/packages/babel-types/src/retrievers.js
index c4d7647b4af9..2395ba30467a 100644
--- a/packages/babel-types/src/retrievers.js
+++ b/packages/babel-types/src/retrievers.js
@@ -10,17 +10,17 @@ export function getBindingIdentifiers(
   outerOnly?: boolean
 ): Object {
   let search = [].concat(node);
-  let ids    = Object.create(null);
+  const ids    = Object.create(null);
 
   while (search.length) {
-    let id = search.shift();
+    const id = search.shift();
     if (!id) continue;
 
-    let keys = t.getBindingIdentifiers.keys[id.type];
+    const keys = t.getBindingIdentifiers.keys[id.type];
 
     if (t.isIdentifier(id)) {
       if (duplicates) {
-        let _ids = ids[id.name] = ids[id.name] || [];
+        const _ids = ids[id.name] = ids[id.name] || [];
         _ids.push(id);
       } else {
         ids[id.name] = id;
@@ -48,7 +48,7 @@ export function getBindingIdentifiers(
 
     if (keys) {
       for (let i = 0; i < keys.length; i++) {
-        let key = keys[i];
+        const key = keys[i];
         if (id[key]) {
           search = search.concat(id[key]);
         }
diff --git a/packages/babel-types/src/validators.js b/packages/babel-types/src/validators.js
index 78ea84cb40f8..183a304a9bdf 100644
--- a/packages/babel-types/src/validators.js
+++ b/packages/babel-types/src/validators.js
@@ -10,11 +10,11 @@ import { BLOCK_SCOPED_SYMBOL } from "./constants";
  */
 
 export function isBinding(node: Object, parent: Object): boolean {
-  let keys = getBindingIdentifiers.keys[parent.type];
+  const keys = getBindingIdentifiers.keys[parent.type];
   if (keys) {
     for (let i = 0; i < keys.length; i++) {
-      let key = keys[i];
-      let val = parent[key];
+      const key = keys[i];
+      const val = parent[key];
       if (Array.isArray(val)) {
         if (val.indexOf(node) >= 0) return true;
       } else {
@@ -73,7 +73,7 @@ export function isReferenced(node: Object, parent: Object): boolean {
     case "ArrowFunctionExpression":
     case "FunctionDeclaration":
     case "FunctionExpression":
-      for (let param of (parent.params: Array<any>)) {
+      for (const param of (parent.params: Array<any>)) {
         if (param === node) return false;
       }
 
@@ -254,7 +254,7 @@ export function isNodesEquivalent(a, b) {
 
   const fields = Object.keys(t.NODE_FIELDS[a.type] || a.type);
 
-  for (let field of fields) {
+  for (const field of fields) {
     if (typeof a[field] !== typeof b[field]) {
       return false;
     }
diff --git a/packages/babel-types/test/cloning.js b/packages/babel-types/test/cloning.js
index 817a4e1ec467..1280f94bd03f 100644
--- a/packages/babel-types/test/cloning.js
+++ b/packages/babel-types/test/cloning.js
@@ -1,24 +1,24 @@
-let t = require("../lib");
-let assert = require("assert");
-let parse = require("babylon").parse;
+const t = require("../lib");
+const assert = require("assert");
+const parse = require("babylon").parse;
 
 suite("cloning", function () {
   suite("clone", function () {
     it("should handle undefined", function () {
-      let node = undefined;
-      let cloned = t.clone(node);
+      const node = undefined;
+      const cloned = t.clone(node);
       assert(cloned === undefined);
     });
 
     it("should handle null", function () {
-      let node = null;
-      let cloned = t.clone(node);
+      const node = null;
+      const cloned = t.clone(node);
       assert(cloned === null);
     });
 
     it("should handle simple cases", function () {
-      let node = t.arrayExpression([null, t.identifier("a")]);
-      let cloned = t.clone(node);
+      const node = t.arrayExpression([null, t.identifier("a")]);
+      const cloned = t.clone(node);
       assert(node !== cloned);
       assert(t.isNodesEquivalent(node, cloned) === true);
     });
@@ -26,42 +26,42 @@ suite("cloning", function () {
 
   suite("cloneDeep", function () {
     it("should handle undefined", function () {
-      let node = undefined;
-      let cloned = t.cloneDeep(node);
+      const node = undefined;
+      const cloned = t.cloneDeep(node);
       assert(cloned === undefined);
     });
 
     it("should handle null", function () {
-      let node = null;
-      let cloned = t.cloneDeep(node);
+      const node = null;
+      const cloned = t.cloneDeep(node);
       assert(cloned === null);
     });
 
     it("should handle simple cases", function () {
-      let node = t.arrayExpression([null, t.identifier("a")]);
-      let cloned = t.cloneDeep(node);
+      const node = t.arrayExpression([null, t.identifier("a")]);
+      const cloned = t.cloneDeep(node);
       assert(node !== cloned);
       assert(t.isNodesEquivalent(node, cloned) === true);
     });
 
     it("should handle full programs", function () {
-      let node = parse("1 + 1");
-      let cloned = t.cloneDeep(node);
+      const node = parse("1 + 1");
+      const cloned = t.cloneDeep(node);
       assert(node !== cloned);
       assert(t.isNodesEquivalent(node, cloned) === true);
     });
 
     it("should handle complex programs", function () {
-      let program = "'use strict'; function lol() { wow();return 1; }";
-      let node = parse(program);
-      let cloned = t.cloneDeep(node);
+      const program = "'use strict'; function lol() { wow();return 1; }";
+      const node = parse(program);
+      const cloned = t.cloneDeep(node);
       assert(node !== cloned);
       assert(t.isNodesEquivalent(node, cloned) === true);
     });
 
     it("should handle missing array element", function () {
-      let node = parse("[,0]");
-      let cloned = t.cloneDeep(node);
+      const node = parse("[,0]");
+      const cloned = t.cloneDeep(node);
       assert(node !== cloned);
       assert(t.isNodesEquivalent(node, cloned) === true);
     });
diff --git a/packages/babel-types/test/validators.js b/packages/babel-types/test/validators.js
index c269614f48bc..b6f6f3178211 100644
--- a/packages/babel-types/test/validators.js
+++ b/packages/babel-types/test/validators.js
@@ -1,14 +1,14 @@
-let t = require("../lib");
-let assert = require("assert");
-let parse = require("babylon").parse;
+const t = require("../lib");
+const assert = require("assert");
+const parse = require("babylon").parse;
 
 suite("validators", function () {
   suite("isNodesEquivalent", function () {
     it("should handle simple cases", function () {
-      let mem = t.memberExpression(t.identifier("a"), t.identifier("b"));
+      const mem = t.memberExpression(t.identifier("a"), t.identifier("b"));
       assert(t.isNodesEquivalent(mem, mem) === true);
 
-      let mem2 = t.memberExpression(t.identifier("a"), t.identifier("c"));
+      const mem2 = t.memberExpression(t.identifier("a"), t.identifier("c"));
       assert(t.isNodesEquivalent(mem, mem2) === false);
     });
 
@@ -18,11 +18,11 @@ suite("validators", function () {
     });
 
     it("should handle complex programs", function () {
-      let program = "'use strict'; function lol() { wow();return 1; }";
+      const program = "'use strict'; function lol() { wow();return 1; }";
 
       assert(t.isNodesEquivalent(parse(program), parse(program)) === true);
 
-      let program2 = "'use strict'; function lol() { wow();return -1; }";
+      const program2 = "'use strict'; function lol() { wow();return -1; }";
 
       assert(t.isNodesEquivalent(parse(program), parse(program2)) === false);
     });