other

A Set object, or set-like object.

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

In mathematical notation, subset is defined as:

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

And using Venn diagram:

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.

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

• Polyfill of Set.prototype.isSubsetOf in core-js
• Set.prototype.difference()
• Set.prototype.intersection()
• Set.prototype.isDisjointFrom()
• Set.prototype.isSupersetOf()
• Set.prototype.symmetricDifference()
• Set.prototype.union()