Weaksets

// In contrast to the Set which can be a collection of anything, Weakset is only a collection of objects and not of arbitrary values of any type. // The WeakSet is weak: References to objects in the collection are held weakly: if there is no other reference to an object stored in the WeakSet, they can be garbage collected. // Also, WeakSets are not enumerable and so you don’t get enumeration methods like .forEach, .clear etc. var ws = new WeakSet(); var obj = {}; var foo = {}; ws.add(window); ws.add(obj); ws.has(window); // true ws.has(foo); // false, foo has not been added to the set ws.delete(window); // removes window from the set ws.has(window); // false, window has been removed