WeakSet : Object

WeakSets are a collection of %%/Object|Objects%% where each
object can only appear once in the set similar to %%/Set|Set%%.
Unlike Set, WeakSet does not allow iterating over its values.
If WeakSet would be the only object holding on to the value, 
the value will be released from memory.

The values stored in WeakSet cannot be primitive values
(%%/Boolean|Boolean%%,
%%/Number|Number%%,
%%/String|String%%, 
or
%%/undefined|undefined%%).

Version:
ECMAScript 2015

Spec:
http://www.ecma-international.org/ecma-262/6.0/#sec-weakset-objects

----
new WeakSet() : WeakSet

Creates an empty WeakSet.

<example>
var value1 = { foo: 1 };
var value2 = { bar: ['a', 'b', 'c'] };

var x = new WeakSet();
x.add(value1);

console.log(x.has(value1));
console.log(x.has(value2));

</example>

Spec:
http://www.ecma-international.org/ecma-262/6.0/#sec-weakset-iterable

----
new WeakSet(iterable : Object) : WeakSet

Creates a WeakSet by iterating over **iterable** and adding
each value to the WeakSet.

<example>
var value1 = { foo: 1 };
var value2 = { bar: ['a', 'b', 'c'] };

var fromArray = new WeakSet([value1, value2]);

console.log(fromArray.has(value1));
console.log(fromArray.has(value2));

var generator = function*() {
  yield value1;
  yield value2;
};

var fromGenerator = new WeakSet(generator());

console.log(fromGenerator.has(value1));
console.log(fromGenerator.has(value2));
</example>

Spec:
http://www.ecma-international.org/ecma-262/6.0/#sec-weakset-iterable

----
prototype.add(value : Object) : Set

Stores **value** in **this**. If **value** is already stored,
there is no change to the WeakSet.
Returns **this**.

<example>
var value1 = { foo: 1 };
var value2 = { bar: ['a', 'b', 'c'] };

var x = new WeakSet([value1]);

console.log(x.has(value1));
console.log(x.has(value2));

x.add(value2);
console.log(x.has(value2));
</example>

Spec:
http://www.ecma-international.org/ecma-262/6.0/#sec-weakset.prototype.add

----
prototype.delete(value : Object) : Boolean

Removes **value** from **this**.
Returns **true** if **value** was in **this** before deleting it.

<example>
var value1 = { foo: 1 };
var value2 = { bar: ['a', 'b', 'c'] };

var x = new WeakSet([value1, value2]);

console.log('has value1 = ' + x.has(value1));
console.log('has value2 = ' + x.has(value2));

console.log('deleted value2 = ' + x.delete(value2));

console.log('has value1 = ' + x.has(value1));
console.log('has value2 = ' + x.has(value2));

console.log('deleted value2 = ' + x.delete(value2)); // Already deleted
</example>

Spec:
http://www.ecma-international.org/ecma-262/6.0/#sec-weakset.prototype.delete

----
prototype.has(value : Object) : Boolean

Returns **true** if the set contains **value**.

<example>
var value1 = { foo: 1 };
var value2 = { bar: ['a', 'b', 'c'] };

var x = new WeakSet([value1]);

console.log(x.has(value1));
console.log(x.has(value2));
</example>

Spec:
http://www.ecma-international.org/ecma-262/6.0/#sec-weakset.prototype.has