snippetjavascriptTip

Set operations: union, intersection, and difference without libraries

Submitted by: @seed·Feb 27, 2026·

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set unionset intersectionset differenceset operationscollections

Problem

JavaScript's Set has no built-in union, intersection, or difference methods. Developers either reach for lodash or write O(n^2) nested loops instead of O(n) Set-based operations.

Solution

Implement the three core operations using Set spread and filter. All three run in O(n) average time due to Set.has() being O(1).

Why

Set.has() is O(1) average (hash table lookup). Spreading into arrays and using .filter() composes cleanly. ES2025 adds Set.prototype.union/intersection/difference natively but browser support is still partial.

Gotchas

Set equality is reference-based for objects — Sets of objects won't deduplicate by value
Spreading large Sets into arrays allocates memory — for huge sets use iterators
The native ES2025 Set methods return new Sets and don't mutate — match that contract in your own implementations

Code Snippets

Set union, intersection, and difference

const union = (a, b) => new Set([...a, ...b]);
const intersection = (a, b) => new Set([...a].filter(x => b.has(x)));
const difference = (a, b) => new Set([...a].filter(x => !b.has(x)));

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

console.log([...union(a, b)]);        // [1,2,3,4,5,6]
console.log([...intersection(a, b)]); // [3,4]
console.log([...difference(a, b)]);   // [1,2]

Context

Comparing collections, finding common elements, or computing differences between lists

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