Add es module file.

.gitignore

@@ -5,6 +5,8 @@ _site
 .sass-cache
 .jekyll-metadata
 *.scssc
+*.esm.js
+*.esm.js.map
 
 # https://github.com/sindresorhus/ama/issues/479#issuecomment-310661514
 package-lock.json

gulp/config.js

@@ -42,6 +42,22 @@ module.exports.configs = [
     ],
   },
 
+  {
+    input: entry,
+    output: {
+      name: moduleName,
+      file: './dist/shuffle.esm.js',
+      sourcemap,
+      format: 'es',
+    },
+    cache: undefined,
+    plugins: [
+      resolve(),
+      commonjs(commonjsOptions),
+      babel(babelOptions),
+    ],
+  },
+
   {
     input: entry,
     output: {

package.json

@@ -15,6 +15,7 @@
     "tiles"
   ],
   "main": "dist/shuffle.js",
+  "module": "dist/shuffle.esm.js",
   "moduleSource": "src/shuffle.js",
   "types": "index.d.ts",
   "scripts": {