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 WeakMap
s in the WeakMap object guide (under Keyed collections).
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 WeakMap
s 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 the references are weak, WeakMap
keys are not enumerable. There is no method to obtain 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
.
Constructor
WeakMap()
- Creates a new
WeakMap
object.
Instance methods
WeakMap.prototype.delete(key)
- Removes any value associated to the
key
.WeakMap.prototype.has(key)
will returnfalse
afterwards. WeakMap.prototype.get(key)
- Returns the value associated to the
key
, orundefined
if there is none. WeakMap.prototype.has(key)
- Returns a Boolean asserting whether a value has been associated to the
key
in theWeakMap
object or not. WeakMap.prototype.set(key, value)
- Sets the
value
for thekey
in theWeakMap
object. Returns theWeakMap
object.
Examples
Using WeakMap
const wm1 = new WeakMap(), wm2 = new WeakMap(), wm3 = new WeakMap(); const 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
Specification |
---|
ECMAScript (ECMA-262) The definition of 'WeakMap' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
WeakMap | Chrome Full support 36 | Edge Full support 12 | Firefox Full support 6 | IE Full support 11 | Opera Full support 23 | Safari Full support 8 | WebView Android Full support 37 | Chrome Android Full support 36 | Firefox Android Full support 6 | Opera Android Full support 24 | Safari iOS Full support 8 | Samsung Internet Android Full support 3.0 | nodejs
Full support
0.12
|
WeakMap() constructor | Chrome Full support 36 | Edge Full support 12 | Firefox Full support 6 | IE Full support 11 | Opera Full support 23 | Safari Full support 8 | WebView Android Full support 37 | Chrome Android Full support 36 | Firefox Android Full support 6 | Opera Android Full support 24 | Safari iOS Full support 8 | Samsung Internet Android Full support 3.0 | nodejs
Full support
0.12
|
clear | Chrome No support 36 — 43 | Edge No support No | Firefox No support 20 — 46 | IE Full support 11 | Opera No support 25 — 30 | Safari No support 8 — 9 | WebView Android No support 37 — 43 | Chrome Android No support 36 — 43 | Firefox Android No support 20 — 46 | Opera Android No support 25 — 30 | Safari iOS No support 8 — 9 | Samsung Internet Android No support 3.0 — 4.0 | nodejs No support 0.12 — 4.0.0 |
delete | Chrome Full support 36 | Edge Full support 12 | Firefox
Full support
6
| IE Full support 11 | Opera Full support 23 | Safari Full support 8 | WebView Android Full support 37 | Chrome Android Full support 36 | Firefox Android
Full support
6
| Opera Android Full support 24 | Safari iOS Full support 8 | Samsung Internet Android Full support 3.0 | nodejs
Full support
0.12
|
get | Chrome Full support 36 | Edge Full support 12 | Firefox
Full support
6
| IE Full support 11 | Opera Full support 23 | Safari Full support 8 | WebView Android Full support 37 | Chrome Android Full support 36 | Firefox Android
Full support
6
| Opera Android Full support 24 | Safari iOS Full support 8 | Samsung Internet Android Full support 3.0 | nodejs
Full support
0.12
|
has | Chrome Full support 36 | Edge Full support 12 | Firefox
Full support
6
| IE Full support 11 | Opera Full support 23 | Safari Full support 8 | WebView Android Full support 37 | Chrome Android Full support 36 | Firefox Android
Full support
6
| Opera Android Full support 24 | Safari iOS Full support 8 | Samsung Internet Android Full support 3.0 | nodejs
Full support
0.12
|
set | Chrome Full support 36 | Edge Full support 12 | Firefox
Full support
6
| IE
Partial support
11
| Opera Full support 23 | Safari Full support 8 | WebView Android Full support 37 | Chrome Android Full support 36 | Firefox Android
Full support
6
| Opera Android Full support 24 | Safari iOS Full support 8 | Samsung Internet Android Full support 3.0 | nodejs
Full support
0.12
|
Legend
- Full support
- Full support
- Partial support
- Partial support
- No support
- No support
- Non-standard. Expect poor cross-browser support.
- Non-standard. Expect poor cross-browser support.
- Deprecated. Not for use in new websites.
- Deprecated. Not for use in new websites.
- See implementation notes.
- See implementation notes.
- User must explicitly enable this feature.
- User must explicitly enable this feature.