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