1.1.0 • Public • Published

Polyfill for WeakMap, Map, WeakSet, and Set

The EcmaScript 6 (Harmony) standard includes four new very useful data structures: WeakMap, Map, WeakSet, and Set. Some more recent browsers have this functionality already; but until all do, you can use this polyfill.

As far as my research shows, this is the only polyfill for all four new data structures (including WeakSet).

Written with love in TypeScript (structs.ts), compiled to plain old JavaScript (ES3) (structs.js), and minified for your convenience (structs.min.js).

License: MIT


In the browser, the polyfill automatically attaches appropriate constructor functions to window if they are missing. So you can just call, e.g., new WeakMap(). As a true polyfill, I've attempted to replicate the exact API in the ES6.

For Node, install through npm:

$ npm install es6-structs

Then in your JS:

var structs = require('es6-structs');
var st = new structs.Set();
var mp = new structs.Map();
var wm = new structs.WeakMap();
var ws = new structs.WeakSet();


Please reference the official literature:


(1) Lookup is O(1).

(2) Objects are mutated when added to the data structures; a new property is added, __structs_uid__, constituting a unique id. On browsers supporting Object.defineProperty(), the property is added such that it is non-enumerable -- that is, such that it doesn't occur in a for ... in loop. However, where this is not supported (such as <IE8), the property is enumerable.

(3) The WeakMap here is leaky in that it has "weak" references to keys but retains values. This means that keys will be GC'ed while values will not. This differs from the ES6 specification in that WeakMap entries are supposed to be deleted whenever keys are GC'ed.

(4) The WeakSet here does not retain its values. However, it is unaware of whether a value was GC'ed or not.




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