Set.prototype.isSubsetOf()

The isSubsetOf() method of Set instances takes a set and returns a boolean indicating if all elements of this set are in the given set.

Syntax

isSubsetOf(other)

Parameters

other: A Set object, or set-like object.

Return value

true if all elements in this set are also in the other set, and false otherwise.

Description

In mathematical notation, subset is defined as:

A\subseteq B \Leftrightarrow \forall x\in A,\,x\in B

And using Venn diagram:

A Venn diagram with two circles. A is a subset of B because A is completely contained in B.

Note: The subset relationship is not proper subset, which means isSubsetOf() returns true if this and other contain the same elements.

isSubsetOf() accepts set-like objects as the other parameter. It requires this to be an actual Set instance, because it directly retrieves the underlying data stored in this without invoking any user code. Then, its behavior depends on the sizes of this and other:

If there are more elements in this than other.size, then it directly returns false.
Otherwise, it iterates over the elements in this, and returns false if any element e in this causes other.has(e) to return a falsy value. Otherwise, it returns true.

Examples

undefined

Using isSubsetOf()

The set of multiples of 4 (<20) is a subset of even numbers (<20):

const fours = new Set([4, 8, 12, 16]);
const evens = new Set([2, 4, 6, 8, 10, 12, 14, 16, 18]);
console.log(fours.isSubsetOf(evens)); // true

The set of prime numbers (<20) is not a subset of all odd numbers (<20), because 2 is prime but not odd:

const primes = new Set([2, 3, 5, 7, 11, 13, 17, 19]);
const odds = new Set([3, 5, 7, 9, 11, 13, 15, 17, 19]);
console.log(primes.isSubsetOf(odds)); // false

Equivalent sets are subsets of each other:

const set1 = new Set([1, 2, 3]);
const set2 = new Set([1, 2, 3]);
console.log(set1.isSubsetOf(set2)); // true
console.log(set2.isSubsetOf(set1)); // true

Specifications

ECMAScript® 2027 Language Specification