what is weakset in javascript code example

Example: weakset in es6

/*
WeakSet() is as same as Set()
Similarities:
	1.WeakSet() has all methods that Set() has.
    2.WeakSet() Donot contain Duplicate Values as Set().
Differences:
    3.WeakSet Values are Non Enumerated
    4.WeakSet Value can only be Empty Objects
*/

var weak = new WeakSet();
weak.add({});
var a={};
weak.add(a);