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

The main differences to the Set object are:

• WeakSets are collections of objects and symbols 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 a value stored in the WeakSet 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 object
function 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, 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.

WeakSet()

Creates a new WeakSet object.

These properties are defined on WeakSet.prototype and shared by all WeakSet instances.

WeakSet.prototype.constructor

The constructor function that created the instance object. For WeakSet instances, the initial value is the WeakSet constructor.

WeakSet.prototype[@@toStringTag]

The initial value of the @@toStringTag property is the string "WeakSet". This property is used in Object.prototype.toString().

WeakSet.prototype.add()

Appends value to the WeakSet object.

WeakSet.prototype.delete()

Removes value from the WeakSet. WeakSet.prototype.has(value) will return false afterwards.

WeakSet.prototype.has()

Returns a boolean asserting whether value is present in the WeakSet object or not.

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.

• Polyfill of WeakSet in core-js
• Map
• Set
• WeakMap