Set.prototype.isSubsetOf()

The isSubsetOf() method of {{jsxref("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 {{jsxref("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:

And using Venn diagram:

[!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 {{jsxref("this")}} to be an actual {{jsxref("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#

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#

{{Specifications}}

Browser compatibility#

{{Compat}}

See also#

• Polyfill of Set.prototype.isSubsetOf in core-js
• es-shims polyfill of Set.prototype.isSubsetOf
• {{jsxref("Set.prototype.difference()")}}
• {{jsxref("Set.prototype.intersection()")}}
• {{jsxref("Set.prototype.isDisjointFrom()")}}
• {{jsxref("Set.prototype.isSupersetOf()")}}
• {{jsxref("Set.prototype.symmetricDifference()")}}
• {{jsxref("Set.prototype.union()")}}