WeakMap : Object

WeakMaps allow associating keys and values similar to %%/Map|Map%%
except WeakMap does not allow iterating over its keys or values.
If WeakMap would be the only object holding on to the key/value
pair, the pair will be released from memory.
The keys 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-weakmap-objects

----
new WeakMap() : WeakMap

Creates an empty WeakMap.

<example>
var key1 = { foo: 1 };
var key2 = { bar: ['a', 'b', 'c'] };

var x = new WeakMap([[key1, 'value 1'], [key2, 'value 2']]);

console.log(x.get(key1));
console.log(x.get(key2));
</example>

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

----
new WeakMap(iterable : Object) : WeakMap

Creates a WeakMap by iterating over **iterable** and using
the first element of the iterator value as the key and
second element as the value.

<example>
var key1 = { foo: 1 };
var key2 = { bar: ['a', 'b', 'c'] };

var fromArray = new WeakMap([[key1, 'array0'], [key2, 'array1']]);

console.log(fromArray.get(key1));
console.log(fromArray.get(key2));
console.log();

var generator = function*() {
  yield [key1, 'generator0'];
  yield [key2, 'generator1'];
};

var fromGenerator = new WeakMap(generator());
console.log(fromGenerator.get(key1));
console.log(fromGenerator.get(key2));
</example>

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

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

Removes **key** and its corresponding value from **this**.
Returns **true** if **key** was in **this** before deleting it.

<example>
var key1 = { foo: 1 };
var key2 = { bar: ['a', 'b', 'c'] };

var x = new WeakMap([[key1, 'value 1'], [key2, 'value 2']]);

console.log(x.get(key1));
console.log(x.get(key2));

x.delete(key1);

console.log(x.get(key1));
console.log(x.get(key2));
</example>

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

----
prototype.get(key : Object) : Object

Returns the value stored for **key**. If no value is stored,
returns %%/undefined|**undefined**%%.
See also %%#has|**has()**%% and %%#set|**set()**%%.

<example>
var key1 = { foo: 1 };
var key2 = { bar: ['a', 'b', 'c'] };

var x = new WeakMap([[key1, 'value 1'], [key2, 'value 2']]);

console.log(x.get(key1));
console.log(x.get(key2));

// get returns undefined for keys not in the WeakMap
var key3 = { language: 'JavaScript' };
console.log(x.get(key3));
</example>

Spec:
http://www.ecma-international.org/ecma-262/6.0/#sec-weakmap.prototype.get

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

Returns **true** if the map has a value stored for **key**.

<example>
var key1 = { foo: 1 };
var key2 = { bar: ['a', 'b', 'c'] };

var x = new WeakMap();

x.set(key1, undefined);

// has() lets you check if a value is stored, even if 
// the value is undefined
console.log(x.has(key1) + ' ' + x.get(key1));
console.log(x.has(key2) + ' ' + x.get(key2));
</example>

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

----
prototype.set(key : Object, value : Object) : WeakMap

Stores **value** in **this** at the specified **key**.  
**key** cannot be a primitive value 
(%%/Boolean|Boolean%%,
%%/Number|Number%%,
%%/String|String%%, 
or
%%/undefined|undefined%%).
If a value is already stored for that **key**, it is replaced with
**value**.
Returns **this**.
See also %%#get|**get()**%% and %%#has|**has()**%%.

<example>
var key1 = { foo: 1 };
var key2 = { bar: ['a', 'b', 'c'] };

var x = new WeakMap([[key1, 'value 1'], [key2, 'value 2']]);

console.log(x.get(key1));

x.set(key1, 'a different value');

console.log(x.get(key1));
</example>

Spec:
http://www.ecma-international.org/ecma-262/6.0/#sec-weakmap.prototype.set