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.

Properties

WeakSet.length

The value of the length property is 0.

WeakSet.prototype

Represents the prototype for the WeakSet constructor. Allows the addition of properties to all WeakSet objects.

WeakSet instances

All WeakSet instances inherit from WeakSet.prototype.

Properties

WeakSet.prototype.constructor

Returns the function that created an instance's prototype. This is the WeakSet function by default.

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.

WeakSet.prototype.clear()

Removes all elements from the WeakSet object.

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 compatibility

See also

• Map
• Set
• WeakMap