JavaScript WeakMap : Object

constructor

WeakMaps allow associating keys and values similar to 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, Number, String, or undefined).

ECMAScript 2015 Spec

Constructors

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));

Run

Results:

Spec

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));

Run

Results:

Spec

Instance Methods

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));

Run

Results:

Spec

get(key : Object) : Object

Returns the value stored for key. If no value is stored, returns undefined. See also has() and 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));

Run

Results:

Spec

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));

Run

Results:

Spec

set(key : Object, value : Object) : WeakMap

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

Run

Results:

Spec

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