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


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){
    _refs = new WeakSet();

  // Avoid infinite recursion

  if("object" === typeof 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.


Creates a new WeakSet object.

Instance methods

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


Using the WeakSet object

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


Browser compatibility

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See also