How can I check if an array is a superset of another array in JavaScript?

A superset is a set that contains all the elements of another set, and possibly more. Knowing how to check if an array is a superset of another array might come in handy from time to time.

As the mathematical definition refers to a set, duplicates are not considered. This is why using a Set object is a good idea. Combining it with Array.prototype.every() and Set.prototype.has() is all that's needed to perform the check.

Checking for a subset is as simple as reversing the order of the arguments. Note that you can use this function for any type of iterable, not just arrays.

const superset = (a, b) => {
  const sA = new Set(a), sB = new Set(b);
  return [...sB].every(v => sA.has(v));
};

const subset = (a, b) => superset(b, a);

superset([1, 2, 3, 4], [1, 2, 2]); // true
superset([1, 2, 3, 4], [1, 3, 5]); // false

subset(new Set([1, 2]), new Set([1, 2, 3, 4])); // true
subset(new Set([1, 5]), new Set([1, 2, 3, 4])); // false

[!NOTE]

Future versions of ECMAScript may include a Set.prototype.isSupersetOf() and Set.prototype.isSubsetOf(), which will make this code snippet obsolete.