WeakMap

The WeakMap object is a collection of key/value pairs in which the keys are weakly referenced.  The keys must be objects and the values can be arbitrary values.

You can learn more about WeakMaps in the section WeakMap object in Keyed collections.

Syntax

new WeakMap([iterable])

Parameters

iterable

Iterable is an Array or other iterable object whose elements are key-value pairs (2-element Arrays). Each key-value pair will be added to the new WeakMap. null is treated as undefined.

Description

Keys of WeakMaps are of the type Object only. Primitive data types as keys are not allowed (e.g. a Symbol can't be a WeakMap key).

Why WeakMap?

A map API could be implemented in JavaScript with two arrays (one for keys, one for values) shared by the four API methods.  Setting elements on this map would involve pushing a key and value onto the end of each of those arrays simultaneously.  As a result, the indices of the key and value would correspond to both arrays.  Getting values from the map would involve iterating through all keys to find a match, then using the index of this match to retrieve the corresponding value from the array of values. 

Such an implementation would have two main inconveniences. The first one is an O(n) set and search (n being the number of keys in the map) since both operations must iterate through the list of keys to find a matching value. The second inconvenience is a memory leak because the arrays ensure that references to each key and each value are maintained indefinitely. These references prevent the keys from being garbage collected, even if there are no other references to the object.  This would also prevent the corresponding values from being garbage collected.

By contrast, native WeakMaps hold "weak" references to key objects, which means that they do not prevent garbage collection in case there would be no other reference to the key object.  This also avoids preventing garbage collection of values in the map.  Native WeakMaps can be particularly useful constructs when mapping keys to information about the key that is valuable only if the key has not been garbage collected.

Because of references being weak, WeakMap keys are not enumerable (i.e. there is no method giving you a list of the keys). If they were, the list would depend on the state of garbage collection, introducing non-determinism. If you want to have a list of keys, you should use a Map.

Properties

WeakMap.length

The value of the length property is 0.

WeakMap.prototype

Represents the prototype for the WeakMap constructor. Allows the addition of properties to all WeakMap objects.

WeakMap instances

All WeakMap instances inherit from WeakMap.prototype.

Properties

WeakMap.prototype.constructor

Returns the function that created an instance's prototype. This is the WeakMap function by default.

Methods

WeakMap.prototype.delete(key)

Removes any value associated to the key. WeakMap.prototype.has(key) will return false afterwards.

WeakMap.prototype.get(key)

Returns the value associated to the key, or undefined if there is none.

WeakMap.prototype.has(key)

Returns a Boolean asserting whether a value has been associated to the key in the WeakMap object or not.

WeakMap.prototype.set(key, value)

Sets the value for the key in the WeakMap object. Returns the WeakMap object.

WeakMap.prototype.clear()

Removes all key/value pairs from the WeakMap object. Note that it is possible to implement a WeakMap-like object that has a .clear() method by encapsulating a WeakMap object that hasn't it (see example on page WeakMap)

Examples

Using WeakMap

var wm1 = new WeakMap(),
    wm2 = new WeakMap(),
    wm3 = new WeakMap();
var o1 = {},
    o2 = function() {},
    o3 = window;

wm1.set(o1, 37);
wm1.set(o2, 'azerty');
wm2.set(o1, o2); // a value can be anything, including an object or a function
wm2.set(o3, undefined);
wm2.set(wm1, wm2); // keys and values can be any objects. Even WeakMaps!

wm1.get(o2); // "azerty"
wm2.get(o2); // undefined, because there is no key for o2 on wm2
wm2.get(o3); // undefined, because that is the set value

wm1.has(o2); // true
wm2.has(o2); // false
wm2.has(o3); // true (even if the value itself is 'undefined')

wm3.set(o1, 37);
wm3.get(o1); // 37

wm1.has(o1); // true
wm1.delete(o1);
wm1.has(o1); // false

Implementing a WeakMap-like class with a .clear() method

class ClearableWeakMap {
  constructor(init) {
    this._wm = new WeakMap(init)
  }
  clear() {
    this._wm = new WeakMap()
  }
  delete(k) {
    return this._wm.delete(k)
  }
  get(k) {
    return this._wm.get(k)
  }
  has(k) {
    return this._wm.has(k)
  }
  set(k, v) {
    this._wm.set(k, v)
    return this
  }
}

Specifications

Browser compatibility

See also

• WeakMap in the JavaScript guide
• Hiding Implementation Details with ECMAScript 6 WeakMaps
• Map
• Set
• WeakSet