ECMAScript6 WeakMap polyfill


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
  • 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).
$ 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);        //  
map.has(obj);        // false 
map.set(obj, 'bar'); // map 
map.clear();         //  
map.has(obj);        // false 


$ npm test