Data Structure: Set in JavaScript

Overview

Sets store unique values of any type and are ideal for deduplication and membership checks.

Sets can contain mixed types (numbers, strings, objects).

Sets are dynamic—no fixed size needed.

Sets preserve insertion order when iterating.

Sets are iterable and work with for...of and forEach.

Set vs Array

• Arrays allow duplicates; Sets do not.
• Arrays use index access; Sets use membership checks and iteration.
• Typical membership tests and deletes in Sets are O(1).

Creating Sets

const set = new Set([1, 2, 3]);
const mixed = new Set([1, '1', { a: 1 }]);
for (const item of set) console.log(item);

Result:

1
2
3

Add & Remove

set.add(4);
set.add(4);

Result:

1
2
3
4

Check membership:

console.log(set.has(4));

Result:

true

Remove values:

set.delete(4);

Result:

1
2
3

Size:

console.log(set.size);

Result:

3

Clear:

set.clear();

Result:

console.log(set.size);
0

Conversions

Array ↔ Set:

const arr = [1, 2, 2, 3];
const s = new Set(arr);
const back = [...s];

Result:

[1, 2, 3]

Deduplicate arrays:

const unique = xs => [...new Set(xs)];
unique([1, 1, 2, 3, 2]);

Result:

[1, 2, 3]

Set Algebra

const union = (A, B) => new Set([...A, ...B]);
const intersection = (A, B) => {
  const out = new Set();
  for (const v of A) if (B.has(v)) out.add(v);
  return out;
};
const difference = (A, B) => {
  const out = new Set();
  for (const v of A) if (!B.has(v)) out.add(v);
  return out;
};
const isSubset = (A, B) => {
  for (const v of A) if (!B.has(v)) return false;
  return true;
};

Examples:

const A = new Set([1, 2, 3]);
const B = new Set([2, 3, 4]);
[...union(A, B)];
[...intersection(A, B)];
[...difference(A, B)];
isSubset(new Set([2, 3]), A);

Result:

[1, 2, 3, 4]
[2, 3]
[1]
true

Big‑O Complexity

Operation	Complexity
has / add / delete	O(1)
clear / size	O(1)
iterate	O(n)

Notes & Pitfalls

• Object equality is by reference
• No index access—iterate or convert to array
• Cache conversions for large sets to avoid repeated cost

Why this still matters when AI writes your code

A Set is the right tool for de-duplication and membership checks, and it is the usual fix for generated O(n²) code: replace array.includes(x) in a loop with set.has(x).

One trap to know about: a Set compares objects by reference, not by value. A Set of { id: 1 } objects will happily hold two equal-looking entries. If you need value uniqueness, key by a primitive such as the id (a Map is often a better fit).

When reviewing AI-written code, scan for repeated membership checks on large arrays and for “unique” logic built from nested loops.

This is part of the Data Structures in JavaScript series. For the bigger picture, read Fundamentals Over Frameworks.