ECMAScript6 WeakMap polyfill

npm install es6-weak-map
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WeakMap collection as specified in ECMAScript6

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

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
  • While clear method is provided, it's not perfectly spec compliant. If some objects were saved as values, they need to be removed via delete. Otherwise they'll remain infinitely attached to key object (that means, they'll be free for GC only if key object was collected as well).


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.clear();         // undefined
map.has(obj);        // false



In your project path:

$ npm install es6-weak-map

You can easily bundle es6-weak-map for browser with modules-webmake

Tests Build Status

$ npm test
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