WeakMap() constructor

The WeakMap() constructor creates WeakMap objects which are a collections 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.

new WeakMap([iterable])

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.

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

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