node package manager


smap.js - A forward polyfill for ES6 Maps

Use ES6 Maps with a bunch of convenience methods. Help improve the ES6 spec!

var map = new Map([['foo', 'bar']]);
map.set(0, 42);
// Filter map by a function 
map.filter(function(key, value, map) {
   return typeof key == 'string';
=> new Map([[0, 42]])
// Merge Maps 
map.merge(new Map([['baz', 'thing']]));
=> new Map([[0, 42], ['baz', 'thing']])
// map.get with a default 
map.fetch('NON_EXISTANT', 'default');
=> 'default'
// Invert map (makes keys->values and vice versa) 
=> new Map([[42, 0], ['thing', 'baz']])
// Destructive filter (inline map delete) 
map.reject(function(key, value, map) {
  return index < 3;
=> false
// Remove all items 
// Check if Map has no items 
=> true


In browsers, include smap-shim.js in your page:

<script type="text/javascript" src=""></script>

You can install this via:

Environment Support

If you also use the es5-shim, you can use this in:

  • IE9+
  • Firefox 13+
  • Google Chrome 21+
  • Safari 4+
  • Opera 12+
  • Node.js 0.8+
  • PhantomJS

Why this project exists

Boris Smus makes an excellent suggestion for moving the web forward: forward polyfills. I hope this project can be the basis for a future proposal to TC39. Please gratuitously discuss Map and WeakMap workings in issues, and add methods/tests with pull requests.

This is for everyone, not just JavaScript gurus. Standards bodies want feedback from developers like you.