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WeakMap() constructor

The WeakMap() creates a WeakMap object, optionally based on a provided Array or other iterable object.

Syntax

new WeakMap()
new WeakMap(iterable)

Note: WeakMap() can only be constructed with new. Attempting to call it without new throws a TypeError.

Parameters

iterable: An Array or other iterable object that implements an @@iterator method that returns an iterator object that produces a two-element array-like object whose first element is a value that will be used as a WeakMap key and whose second element is the value to associate with that key. Each key-value pair will be added to the new WeakMap. null is treated as undefined.

Examples

Using WeakMap

const wm1 = new WeakMap();
const wm2 = new WeakMap();
const wm3 = new WeakMap();
const o1 = {};
const o2 = function () {};
const 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

Specifications

Specification
ECMAScript Language Specification # sec-weakmap-constructor

Browser compatibility

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	Deno	Node.js
`WeakMap`	36	12	6	11	23	8	37	36	6	24	8	3.0	1.0	0.12.0 0.10.0
`iterable_allowed`	38	12	36	No	25	9	38	38	36	25	9	3.0	1.0	0.12.0
`new_required`	36	12	42	11	23	9	37	36	42	24	9	3.0	1.0	0.12.0
`null_allowed`	36	12	37	11	23	8	37	36	37	24	8	3.0	1.0	0.12.0 0.10.0