WeakSets

Whereas a WeakMap holds its keys weakly (but its values strongly), a WeakSet holds its values weakly (there aren’t really keys).

var s = new WeakSet();
var x = { id: 1 },
    y = { id: 2 };
s.add( x );
s.add( y );
x = null;                        // `x` is GC-eligible
y = null;                        // `y` is GC-eligible

Warning: WeakSet values must be objects, not primitive values as is allowed with sets.