What's the difference between ES6 Set and WeakSet?

Question

ECMAScript 6 has these very similar collections: Set and WeakSet. What is the difference between them?

Answer 1

The main difference is that references to objects in Set are strong while references to objects in WeakSet are weak. This means that an object in WeakSet can be garbage collected if there is no other reference to it.

Other differences (or rather side-effects) are:

• Sets can store any value. WeakSets are collections of objects only.
• WeakSet does not have size property.
• WeakSet does not have clear, keys, values, entries, forEach methods.
• WeakSet is not iterable.

Answer 2

Summary:

Weaksets are javascript objects which holds a collection of objects. Due to the nature of a set only one object reference of the same object may occur within the set. A Weakset differs from a normal set in the following ways:

1. Weaksets can only hold objects within its collection, no primitive values (e.g. int, boolean, string) are allowed.
2. References to the objects are held weak. This means that whenever there is no other reference towards the object besides the WeakSet, the object can be be garbage collected (i.e. the JS engine will free the memory which object the reference was pointing to).

Example:

let myWeakSet = new WeakSet();
let obj = {};
myWeakSet.add(obj); 
console.log(myWeakSet.has(obj));

// break the last reference to the object we created earlier
obj = 5;

// false because no other references to the object which the weakset points to
// because weakset was the only object holding a reference it released it and got garbage collected
console.log(myWeakSet.has(obj));