Web/JavaScript/Reference/Global objects/WeakSet

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

Description

WeakSet objects are collections of objects. Just as with Sets, each object in a WeakSet may occur only once; all objects in a WeakSet's collection are unique.

The main differences to the Set object are:

WeakSets are collections of objects only. They cannot contain arbitrary values of any type, as Sets can.
The WeakSet is weak, meaning references to objects in a WeakSet are held weakly. If no other references to an object stored in the WeakSet exist, those objects can be garbage collected.

Note: This also means that there is no list of current objects stored in the collection. WeakSets are not enumerable.

Use case: Detecting circular references

Functions that call themselves recursively need a way of guarding against circular data structures by tracking which objects have already been processed.

WeakSets are ideal for this purpose:

// Execute a callback on everything stored inside an object
function execRecursively(fn, subject, _refs = null){
  if(!_refs)
    _refs = new WeakSet();
  
  // Avoid infinite recursion
  if(_refs.has(subject))
    return;

  fn(subject);
  if("object" === typeof subject){
    _refs.add(subject);
    for(let key in subject)
      execRecursively(fn, subject[key], _refs);
  }
}

const foo = {
  foo: "Foo",
  bar: {
    bar: "Bar"
  }
};

foo.bar.baz = foo; // Circular reference!
execRecursively(obj => console.log(obj), foo);

Here, a WeakSet is created on the first run, and passed along with every subsequent function call (using the internal _refs parameter).

The number of objects or their traversal order is immaterial, so a WeakSet is more suitable (and performant) than a Set for tracking object references, especially if a very large number of objects is involved.

Constructor

WeakSet(): Creates a new WeakSet object.

Instance methods

WeakSet.prototype.add(value): Appends value to the WeakSet object.
WeakSet.prototype.delete(value): Removes value from the WeakSet. WeakSet.prototype.has(value) will return false afterwards.
WeakSet.prototype.has(value): Returns a boolean asserting whether value is present in the WeakSet object or not.

Examples

Using the WeakSet object

const ws = new WeakSet();
const foo = {};
const bar = {};

ws.add(foo);
ws.add(bar);

ws.has(foo);    // true
ws.has(bar);    // true

ws.delete(foo); // removes foo from the set
ws.has(foo);    // false, foo has been removed
ws.has(bar);    // true, bar is retained

Note that foo !== bar. While they are similar objects, they are not the same object. And so they are both added to the set.

Specifications

Specification
ECMAScript (ECMA-262)The definition of 'WeakSet' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`WeakSet`	Chrome Full support 36	Edge Full support 12	Firefox Full support 34	IE No support No	Opera Full support 23	Safari Full support 9	WebView Android Full support 37	Chrome Android Full support 36	Firefox Android Full support 34	Opera Android Full support 24	Safari iOS Full support 9	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`WeakSet()` constructor	Chrome Full support 36	Edge Full support 12	Firefox Full support 34	IE No support No	Opera Full support 23	Safari Full support 9	WebView Android Full support 37	Chrome Android Full support 36	Firefox Android Full support 34	Opera Android Full support 24	Safari iOS Full support 9	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`add`	Chrome Full support 36	Edge Full support 12	Firefox Full support 34	IE No support No	Opera Full support 23	Safari Full support 9	WebView Android Full support 37	Chrome Android Full support 36	Firefox Android Full support 34	Opera Android Full support 24	Safari iOS Full support 9	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`clear` Deprecated'Non-standard'	Chrome No support 36 — 41	Edge No support No	Firefox No support 34 — 46	IE No support No	Opera No support 23 — 28	Safari No support No	WebView Android No support 37 — 41	Chrome Android No support 36 — 41	Firefox Android No support 34 — 46	Opera Android No support 24 — 28	Safari iOS No support No	Samsung Internet Android No support 3.0 — 4.0	nodejs No support No
`delete`	Chrome Full support 36	Edge Full support 12	Firefox Full support 34	IE No support No	Opera Full support 23	Safari Full support 9	WebView Android Full support 37	Chrome Android Full support 36	Firefox Android Full support 34	Opera Android Full support 24	Safari iOS Full support 9	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`has`	Chrome Full support 36	Edge Full support 12	Firefox Full support 34	IE No support No	Opera Full support 23	Safari Full support 9	WebView Android Full support 37	Chrome Android Full support 36	Firefox Android Full support 34	Opera Android Full support 24	Safari iOS Full support 9	Samsung Internet Android Full support 3.0	nodejs Full support 0.12

Legend

Full support: Full 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.