Values of WeakSets must be garbage-collectable. Mostprimitive data types can be arbitrarily created and don't have a lifetime, so they cannot be stored. Objects andnon-registered symbols can be stored because they are garbage-collectable.

The main differences to theSet object are:

• WeakSets are collections ofobjects and symbols only. They cannot contain arbitrary values of any type, asSets can.

• TheWeakSet isweak, meaning references to objects in aWeakSet are heldweakly. If no other references to a value stored in theWeakSet exist, those values can be garbage collected.

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

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 objectfunction execRecursively(fn, subject, _refs = new WeakSet()) {  // Avoid infinite recursion  if (_refs.has(subject)) {    return;  }  fn(subject);  if (typeof subject === "object" && subject) {    _refs.add(subject);    for (const key in subject) {      execRecursively(fn, subject[key], _refs);    }    _refs.delete(subject);  }}const foo = {  foo: "Foo",  bar: {    bar: "Bar",  },};foo.bar.baz = foo; // Circular reference!execRecursively((obj) => console.log(obj), foo);

Here, aWeakSet 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 aWeakSet is more suitable (and performant) than aSet for tracking object references, especially if a very large number of objects is involved.

WeakSet()

Creates a newWeakSet object.

These properties are defined onWeakSet.prototype and shared by allWeakSet instances.

WeakSet.prototype.constructor

The constructor function that created the instance object. ForWeakSet instances, the initial value is theWeakSet constructor.

WeakSet.prototype[Symbol.toStringTag]

The initial value of the[Symbol.toStringTag] property is the string"WeakSet". This property is used inObject.prototype.toString().

WeakSet.prototype.add()

Appendsvalue to theWeakSet object.

WeakSet.prototype.delete()

Removesvalue from theWeakSet.WeakSet.prototype.has(value) will returnfalse afterwards.

WeakSet.prototype.has()

Returns a boolean asserting whethervalue is present in theWeakSet object or not.

const ws = new WeakSet();const foo = {};const bar = {};ws.add(foo);ws.add(bar);ws.has(foo); // truews.has(bar); // truews.delete(foo); // removes foo from the setws.has(foo); // false, foo has been removedws.has(bar); // true, bar is retained

Note thatfoo !== bar. While they are similar objects,they are notthe same object. And so they are both added to the set.

• Polyfill ofWeakSet incore-js
• Map
• Set
• WeakMap