weak-map

WeakMap is a 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.

Purpose and limitation

This shim depends on and modifies ECMAScript 5 property descriptor related methods, Object.defineProperty, Object.getOwnPropertyNames, Object.isExtensible, Object.freeze, and Object.seal.

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. The shim overrides the ECMAScript 5 methods to cover its tracks.

Consider a scenario that only includes a weak map, a key, and a corresponding value through the weak map. With a proper WeakMap, built into the JavaScript engine privy to the internals of the garbage collector, the value would be retained either by the key or the weak map. If either the key or the weak map are elligible for garbage collection, the value is elligible.

This is in contrast to to a plain Map. In a scenario with a map, a key, and a value corresponding to the key through the map, neither the key nor the value will be eligible for garbage collection until the map containing them is elligible. Thus, if a map is used to establish a relationship between ephemeral keys and values, it will accumulate garbage.

This shim does its best to approximate a proper WeakMap without an intimate relationship with the garbage collector. In the same scenario, the value will become elligible for garbage collection if the key is elligible. Unlike a proper weak map, if the weak map shim becomes elligible for garbage collection but the key is retained by something else, the value will be retained. In this scenario, all operations of the weak map take constant time.

However, if the key is frozen, the weak map retains both the key and the value and neither are elligible for collection until the weak map becomes elligible itself. This scenario is unfortunately identical to the behavior of a Map. Additionally, all operations of the weak map suffer linear time.

As stated by Mark Miller in the code:

As with true WeakMaps, in this emulation, a key does not retain maps indexed by that key and (crucially) a map does not retain the keys it indexes. A map by itself also does not retain the values associated with that map.

However, the values associated with a key in some map are retained so long as that key is retained and those associations are not overridden. For example, when used to support membranes, all values exported from a given membrane will live for the lifetime they would have had in the absence of an interposed membrane. Even when the membrane is revoked, all objects that would have been reachable in the absence of revocation will still be reachable, as far as the GC can tell, even though they will no longer be relevant to ongoing computation.

The API implemented here is approximately the API as implemented in FF6.0a1 and agreed to by MarkM, Andreas Gal, and Dave Herman, rather than the offially approved proposal page.

The first difference between the emulation here and that in FF6.0a1 is the presence of non enumerable get___, has___, set___, and delete___} methods on WeakMap instances to represent what would be the hidden internal properties of a primitive implementation. Whereas the FF6.0a1 WeakMap.prototype methods require their this to be a genuine WeakMap instance (i.e., an object of [[Class]] "WeakMap}), since there is nothing unforgeable about the pseudo-internal method names used here, nothing prevents these emulated prototype methods from being applied to non-WeakMaps with pseudo-internal methods of the same names.

Another difference is that our emulated WeakMap.prototype is not itself a WeakMap. A problem with the current FF6.0a1 API is that WeakMap.prototype is itself a WeakMap providing ambient mutability and an ambient communications channel. Thus, if a WeakMap is already present and has this problem, repairES5.js wraps it in a safe wrappper in order to prevent access to this channel. (See PATCH_MUTABLE_FROZEN_WEAKMAP_PROTO in repairES5.js).

This refers to repairES5.js as provided by Google Caja.

Origin and license

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.

This work began with Mark Miller’s proposal for WeakMap to ECMA’s TC-39, where the JavaScript standard is developed.