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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

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
WeakSet 36 12 34 No 23 9
WeakSet() constructor 36 12 34 No 23 9
add 36 12 34 No 23 9
clear 36 — 41 No 34 — 46 No 23 — 28 No
delete 36 12 34 No 23 9
has 36 12 34 No 23 9
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
WeakSet 37 36 34 24 9 3.0
WeakSet() constructor 37 36 34 24 9 3.0
add 37 36 34 24 9 3.0
clear 37 — 41 36 — 41 34 — 46 24 — 28 No 3.0 — 4.0
delete 37 36 34 24 9 3.0
has 37 36 34 24 9 3.0
Server
Node.js
WeakSet 0.12
WeakSet() constructor 0.12
add 0.12
clear No
delete 0.12
has 0.12

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet