JSPM – weak-map@1.0.2

Package Exports

weak-map

This package does not declare an exports field, so the exports above have been automatically detected and optimized by JSPM instead. If any package subpath is missing, it is recommended to post an issue to the original package (weak-map) to support the "exports" field. If that is not possible, create a JSPM override to customize the exports field for this package.

Readme

WeakMap is a new collection slated to be introduced to JavaScript with EcmaScript 6. It provides a mapping from objects to values, but allows any entry to be garbage collected if the key is provably lost.

In order for it to be possible that a key is provably lost, weak maps do not provide a way to access the key list.

This is a Node Packaged Module (NPM) that provides a shim and patcher for missing or broken WeakMap implementations suitable for use in Node.js and browsers that provide the EcmaScript 5 property description interfaces provided that it hosted by a CommonJS loader or bundler like Browserify, Montage, Mr, or Mop.

npm install weak-map --save

var WeakMap = require("weak-map");
var map = new WeakMap();
var key = {};
map.set(key, "Hello, World!");
map.get(key) === "Hello, World!";
key = null;
// "Hello, World!" may be collected

See MDN for the API details.

At time of writing, prototype implementations of WeakMap exist in V8 and Spidermonkey. The prototype is available in Node.js v0.10 with the --harmony_collections V8 option. In v0.8, it was available with --harmony_weakmaps. The purpose of this package is to enable dependees to use weak maps regardless of whether they are implemented by the underlying engine, albeit in a way that leaks memory in some non-obvious cases.

The canonical implementation of WeakMap exists in the Google Caja Subversion repository at http://google-caja.googlecode.com/svn/trunk. It was written by Mark S. Miller. It is released by Google with the Apache 2.0 license. This package is maintained by Kris Kowal.

In a nutshell, the WeakMap shim emulates a WeakMap by adding a hidden property to the key that associates the weak map with the retained object. Thus, in many cases, if the key is garbage collected, the value may be garbage collected. I will leave Mark Miller to evince the details. The shim depends on EcmaScript 5’s API’s to cover its tracks.