Understanding WeakSets

WeakSet is very similar to WeakMap; the values that a WeakSet can hold are only objects and cannot be primitives just like in the case of a WeakMap. The WeakSets are also not enumerable, so you do not have direct access to the values available inside the set.

Let's create a small example and understand the difference between a Set and a WeakSet: 

var set = new Set();
var wset = new WeakSet();

(function() {
  
  var a = {a: 1};
  var b = {b: 2};
  var c = {c: 3};
  var d = {d: 4};
  
  set.add(1).add(2).add(3).add(4);
  wset.add(a).add(b).add(b).add(d);
   
})();

console.dir(set);
console.dir(wset);

One important thing to note is that WeakSet does not accept primitives and can only accept objects similar to the WeakMap keys.

The output of the preceding code is as follows, which is what was expected from the WeakSet. WeakSet does not retain elements beyond the lifespan of the variables that were holding them:

As expected, the WeakSet is empty once the IIFE is terminated.