This is a new technology, part of the ECMAScript 2015 (ES6) standard .
This technology's specification has been finalized, but check the compatibility table for usage and implementation status in various browsers.

The WeakSet object lets you store weakly held objects in a collection.


 new WeakSet([iterable]);


If an iterable object is passed, all of its elements will be added to the new WeakSet. null is treated as undefined.


WeakSet objects are collections of objects. An object in the WeakSet may only occur once; it is unique in the WeakSet's collection.

The main differences to the Set object are:

  • In contrast to Sets, WeakSets are collections of objects only and not of arbitrary values of any type.
  • The WeakSet is weak: References to objects in the collection are held weakly. If there is no other reference to an object stored in the WeakSet, they can be garbage collected. That also means that there is no list of current objects stored in the collection. WeakSets are not enumerable.


The value of the length property is 0.
Represents the prototype for the Set constructor. Allows the addition of properties to all WeakSet objects.

WeakSet instances

All WeakSet instances inherit from WeakSet.prototype.


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


Appends a new element with the given value to the WeakSet object.
Removes the element associated to the value. WeakSet.prototype.has(value) will return false afterwards.
Returns a boolean asserting whether an element is present with the given value in the WeakSet object or not.
Removes all elements from the WeakSet object.


Using the WeakSet object

var ws = new WeakSet();
var obj = {};
var foo = {};


ws.has(window); // true
ws.has(foo);    // false, foo has not been added to the set

ws.delete(window); // removes window from the set
ws.has(window);    // false, window has been removed

ws.clear(); // empty the whole WeakSet


Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'WeakSet' in that specification.
Standard Initial definition.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 36 34 (34) Not supported 23 Not supported
new WeakSet(iterable) 38 34 (34) Not supported 25 Not supported
Constructor argument: new WeakSet(null) (Yes) 37 (37) ? ? ?
Monkey-patched add() in Constructor (Yes) 37 (37) ? ? ?
Feature Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support Not supported 34.0 (34) Not supported Not supported Not supported
new WeakMap(iterable) Not supported 34.0 (34) Not supported Not supported Not supported
Constructor argument: new WeakSet(null) ? (Yes) ? ? ?
Monkey-patched add() in Constructor ? (Yes) ? ? ?

See also

Document Tags and Contributors

Contributors to this page: fscholz, Jeremie, teoli, m_gol, claudepache, phistuck, arai, sanderd17, realityking
Last updated by: fscholz,