JavaScript WeakSet : Object

constructor

WeakSets are a collection of Objects where each object can only appear once in the set similar to 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, Number, String, or undefined).

ECMAScript 2015 Spec

Constructors

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));

Run

Results:

Spec

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));

Run

Results:

Spec

Instance Methods

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));

Run

Results:

Spec

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

Run

Results:

Spec

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));

Run

Results:

Spec

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