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Hash Set

An unordered collection of unique values with O(1) average-case add, delete, and membership test.

Since ES2024 Spec ↗

Syntax

const set = new Set([iterable]);

Parameters

NameTypeRequiredDescription
iterable Iterable<value> | null No An optional iterable of initial values. Duplicate values are silently ignored.

Returns

Set — A new Set instance containing only unique values.

Examples

// Remove duplicates from an array
const nums = [1, 2, 2, 3, 3, 3, 4];
const unique = [...new Set(nums)];
console.log(unique);
Output 
[ 1, 2, 3, 4 ]

// Longest consecutive sequence (O(n) with a Set)
function longestConsecutive(nums) {
  const s = new Set(nums);
  let best = 0;
  for (const n of s) {
    if (s.has(n - 1)) continue; // only start from sequence head
    let len = 1;
    while (s.has(n + len)) len++;
    best = Math.max(best, len);
  }
  return best;
}
console.log(longestConsecutive([100, 4, 200, 1, 3, 2]));
Output 
4

Notes

| Operation | Best | Avg | Worst | Space | |----------|------|------|-------|-------| | Add | O(1) | O(1) | O(n) | O(n) | | Has | O(1) | O(1) | O(n) | O(1) | | Delete | O(1) | O(1) | O(n) | O(1) | JavaScript's `Set` maintains insertion order and supports `forEach`, spread, and `for...of`. Unlike a `Map`, it stores only values (no key-value pairs). Use it for O(1) membership tests instead of `Array.includes()` which is O(n).

See also