ECMAScript6 WeakMap polyfill

Usage no npm install needed!

<script type="module">
  import es6WeakMap from '';


Build status Windows status Transpilation status npm version


WeakMap collection as specified in ECMAScript6

Roughly inspired by Mark Miller's and Kris Kowal's WeakMap implementation.

Differences are:

  • Assumes compliant ES5 environment (no weird ES3 workarounds or hacks)
  • Well modularized CJS style
  • Based on one solution.


  • Will fail on non extensible objects provided as keys


$ npm install es6-weak-map

To port it to Browser or any other (non CJS) environment, use your favorite CJS bundler. No favorite yet? Try: Browserify, Webmake or Webpack


If you want to make sure your environment implements WeakMap, do:


If you'd like to use native version when it exists and fallback to polyfill if it doesn't, but without implementing WeakMap on global scope, do:

var WeakMap = require("es6-weak-map");

If you strictly want to use polyfill even if native WeakMap exists, do:

var WeakMap = require("es6-weak-map/polyfill");


Best is to refer to specification. Still if you want quick look, follow example:

var WeakMap = require("es6-weak-map");

var map = new WeakMap();
var obj = {};

map.set(obj, "foo"); // map
map.get(obj); // 'foo'
map.has(obj); // true
map.delete(obj); // true
map.get(obj); // undefined
map.has(obj); // false
map.set(obj, "bar"); // map
map.has(obj); // false


$ npm test