Set Methods

You spread into arrays, filter, then wrap in a new Set just to find the overlap between two collections. That ritual is over. Set now has seven built-in methods — union, intersection, difference, symmetricDifference, isSubsetOf, isSupersetOf, and isDisjointFrom — shipping in every modern browser since mid-2024.

The seven methods

const a = new Set([1, 2, 3, 4]);
const b = new Set([3, 4, 5, 6]);

a.union(b); // Set {1, 2, 3, 4, 5, 6}
a.intersection(b); // Set {3, 4}
a.difference(b); // Set {1, 2}
a.symmetricDifference(b); // Set {1, 2, 5, 6}

a.isSubsetOf(b); // false
a.isSupersetOf(b); // false
a.isDisjointFrom(b); // false

Usage

const adminPerms = new Set(['read', 'write', 'delete', 'manage-users']);
const editorPerms = new Set(['read', 'write', 'publish']);

// Shared permissions
adminPerms.intersection(editorPerms);
// Set {"read", "write"}

// Every permission across both roles
adminPerms.union(editorPerms);
// Set {"read", "write", "delete", "manage-users", "publish"}

// Admin-only permissions
adminPerms.difference(editorPerms);
// Set {"delete", "manage-users"}

// Does editor have everything admin has?
editorPerms.isSubsetOf(adminPerms); // false

// Tag matching — quick "any overlap?" check
const userInterests = new Set(['react', 'typescript', 'go']);
const postTags = new Set(['rust', 'wasm']);
userInterests.isDisjointFrom(postTags); // true — no match, skip this post

Gotchas

• No chaining shortcut. a.union(b).intersection(c) works, but each call creates an intermediate Set. For complex expressions on large collections, store intermediates explicitly.
• Comparison is SameValueZero. Same rule Set has always used — NaN === NaN is true, but objects compare by reference, not by value.
• No Array input. a.union([3, 4]) throws. Wrap arrays first: a.union(new Set([3, 4])).

Reach for these methods anytime you compare, merge, or diff collections. They replace the spread-filter-has dance with one readable call — and the engine handles the fast path.

Use Remove Duplicates from an Array when you need a unique array, or Group Array by Key to bucket items before comparing groups.