diff --git a/lib/zlib.js b/lib/zlib.js
index 80fe1fd24dab9a..5266c1cfe4a2b1 100644
--- a/lib/zlib.js
+++ b/lib/zlib.js
@@ -93,8 +93,8 @@ const codes = {
 };
 
 const ckeys = ObjectKeys(codes);
-for (var ck = 0; ck < ckeys.length; ck++) {
-  var ckey = ckeys[ck];
+for (let ck = 0; ck < ckeys.length; ck++) {
+  const ckey = ckeys[ck];
   codes[codes[ckey]] = ckey;
 }
 
@@ -132,14 +132,14 @@ function zlibBufferOnError(err) {
 }
 
 function zlibBufferOnEnd() {
-  var buf;
-  var err;
+  let buf;
+  let err;
   if (this.nread >= kMaxLength) {
     err = new ERR_BUFFER_TOO_LARGE();
   } else if (this.nread === 0) {
     buf = Buffer.alloc(0);
   } else {
-    var bufs = this.buffers;
+    const bufs = this.buffers;
     buf = (bufs.length === 1 ? bufs[0] : Buffer.concat(bufs, this.nread));
   }
   this.close();
@@ -231,7 +231,7 @@ const checkRangesOrGetDefault = hideStackFrames(
 
 // The base class for all Zlib-style streams.
 function ZlibBase(opts, mode, handle, { flush, finishFlush, fullFlush }) {
-  var chunkSize = Z_DEFAULT_CHUNK;
+  let chunkSize = Z_DEFAULT_CHUNK;
   // The ZlibBase class is not exported to user land, the mode should only be
   // passed in by us.
   assert(typeof mode === 'number');
@@ -380,7 +380,7 @@ ZlibBase.prototype._destroy = function(err, callback) {
 };
 
 ZlibBase.prototype._transform = function(chunk, encoding, cb) {
-  var flushFlag = this._defaultFlushFlag;
+  let flushFlag = this._defaultFlushFlag;
   // We use a 'fake' zero-length chunk to carry information about flushes from
   // the public API to the actual stream implementation.
   if (typeof chunk[kFlushFlag] === 'number') {
@@ -404,22 +404,22 @@ ZlibBase.prototype._processChunk = function(chunk, flushFlag, cb) {
 };
 
 function processChunkSync(self, chunk, flushFlag) {
-  var availInBefore = chunk.byteLength;
-  var availOutBefore = self._chunkSize - self._outOffset;
-  var inOff = 0;
-  var availOutAfter;
-  var availInAfter;
-
-  var buffers = null;
-  var nread = 0;
-  var inputRead = 0;
+  let availInBefore = chunk.byteLength;
+  let availOutBefore = self._chunkSize - self._outOffset;
+  let inOff = 0;
+  let availOutAfter;
+  let availInAfter;
+
+  let buffers = null;
+  let nread = 0;
+  let inputRead = 0;
   const state = self._writeState;
   const handle = self._handle;
-  var buffer = self._outBuffer;
-  var offset = self._outOffset;
+  let buffer = self._outBuffer;
+  let offset = self._outOffset;
   const chunkSize = self._chunkSize;
 
-  var error;
+  let error;
   self.on('error', function onError(er) {
     error = er;
   });
@@ -438,12 +438,12 @@ function processChunkSync(self, chunk, flushFlag) {
     availOutAfter = state[0];
     availInAfter = state[1];
 
-    var inDelta = (availInBefore - availInAfter);
+    const inDelta = (availInBefore - availInAfter);
     inputRead += inDelta;
 
-    var have = availOutBefore - availOutAfter;
+    const have = availOutBefore - availOutAfter;
     if (have > 0) {
-      var out = buffer.slice(offset, offset + have);
+      const out = buffer.slice(offset, offset + have);
       offset += have;
       if (!buffers)
         buffers = [out];
@@ -528,7 +528,7 @@ function processCallback() {
 
   const have = handle.availOutBefore - availOutAfter;
   if (have > 0) {
-    var out = self._outBuffer.slice(self._outOffset, self._outOffset + have);
+    const out = self._outBuffer.slice(self._outOffset, self._outOffset + have);
     self._outOffset += have;
     self.push(out);
   } else {
@@ -600,11 +600,11 @@ const zlibDefaultOpts = {
 // Base class for all streams actually backed by zlib and using zlib-specific
 // parameters.
 function Zlib(opts, mode) {
-  var windowBits = Z_DEFAULT_WINDOWBITS;
-  var level = Z_DEFAULT_COMPRESSION;
-  var memLevel = Z_DEFAULT_MEMLEVEL;
-  var strategy = Z_DEFAULT_STRATEGY;
-  var dictionary;
+  let windowBits = Z_DEFAULT_WINDOWBITS;
+  let level = Z_DEFAULT_COMPRESSION;
+  let memLevel = Z_DEFAULT_MEMLEVEL;
+  let strategy = Z_DEFAULT_STRATEGY;
+  let dictionary;
 
   if (opts) {
     // windowBits is special. On the compression side, 0 is an invalid value.
@@ -913,8 +913,8 @@ ObjectDefineProperties(module.exports, {
 // These should be considered deprecated
 // expose all the zlib constants
 const bkeys = ObjectKeys(constants);
-for (var bk = 0; bk < bkeys.length; bk++) {
-  var bkey = bkeys[bk];
+for (let bk = 0; bk < bkeys.length; bk++) {
+  const bkey = bkeys[bk];
   if (bkey.startsWith('BROTLI')) continue;
   ObjectDefineProperty(module.exports, bkey, {
     enumerable: false, value: constants[bkey], writable: false