diff --git a/packages/babel-cli/src/babel/dir.js b/packages/babel-cli/src/babel/dir.js
index c6790d5acea7..234eb7c8f60b 100644
--- a/packages/babel-cli/src/babel/dir.js
+++ b/packages/babel-cli/src/babel/dir.js
@@ -145,7 +145,7 @@ export default async function({
   if (cliOptions.watch) {
     const chokidar = util.requireChokidar();
 
-    filenames.forEach(function(filenameOrDir) {
+    filenames.forEach(function(filenameOrDir: string): void {
       const watcher = chokidar.watch(filenameOrDir, {
         persistent: true,
         ignoreInitial: true,
@@ -155,8 +155,8 @@ export default async function({
         },
       });
 
-      ["add", "change"].forEach(function(type) {
-        watcher.on(type, function(filename) {
+      ["add", "change"].forEach(function(type: string): void {
+        watcher.on(type, function(filename: string): void {
           handleFile(
             filename,
             filename === filenameOrDir
diff --git a/packages/babel-cli/src/babel/file.js b/packages/babel-cli/src/babel/file.js
index 23b0307cc884..1ad5d6b84772 100644
--- a/packages/babel-cli/src/babel/file.js
+++ b/packages/babel-cli/src/babel/file.js
@@ -223,7 +223,7 @@ export default async function({
             pollInterval: 10,
           },
         })
-        .on("all", function(type: string, filename: string) {
+        .on("all", function(type: string, filename: string): void {
           if (!util.isCompilableExtension(filename, cliOptions.extensions)) {
             return;
           }
diff --git a/packages/babel-cli/src/babel/options.js b/packages/babel-cli/src/babel/options.js
index 04a648c1fd5a..e68f4dfe8ea8 100644
--- a/packages/babel-cli/src/babel/options.js
+++ b/packages/babel-cli/src/babel/options.js
@@ -313,7 +313,10 @@ function booleanify(val: any): boolean | any {
   return val;
 }
 
-function collect(value, previousValue): Array<string> {
+function collect(
+  value: string | any,
+  previousValue: Array<string>,
+): Array<string> {
   // If the user passed the option with no value, like "babel file.js --presets", do nothing.
   if (typeof value !== "string") return previousValue;