Map is a key → value store where keys can be anything; Set is a collection of unique values. Both keep insertion order and have O(1) average lookups.

Map

JavaScript

const m = new Map();
const objKey = { id: 1 };
m.set("name", "Asha").set(42, "number key").set(objKey, "object key");
console.log(m.get(objKey), m.get(42), m.get("missing"));
console.log(m.has("name"), m.size);
m.delete(42);
console.log(m);
for (const [key, value] of m) console.log(key, "→", value);
console.log([...m.keys()], new Map(Object.entries({ a: 1 })));

Output

object key number key undefined
true 3
Map(2) { 'name' => 'Asha', { id: 1 } => 'object key' }
name → Asha
{ id: 1 } → object key
[ 'name', { id: 1 } ] Map(1) { 'a' => 1 }

Map vs plain object

	`Object`	`Map`
Key types	Strings and symbols (others are stringified)	Any value — objects, functions, `NaN`
Order	Integer keys first, then insertion	Pure insertion order
Size	`Object.keys(o).length`	`m.size`
Iteration	Via `Object.entries`	Directly iterable
Accidental keys	Inherits `toString`, `constructor`, `__proto__` risks	None
Frequent add/delete	Slower	Optimised for it
JSON support	Native	Convert: `Object.fromEntries(m)`

Set

JavaScript

const s = new Set([3, 1, 3, 2, 1]);
s.add(4).add(1);
console.log(s, s.size, s.has(2));
s.delete(3);
console.log([...s]);
console.log([...new Set("mississippi")].join(""));

Output

Set(4) { 3, 1, 2, 4 } 4 true
[ 1, 2, 4 ]
misp

Set operations (ES2025, Node 22+)

JavaScript

const a = new Set([1, 2, 3, 4]);
const b = new Set([3, 4, 5]);
console.log(a.union(b));
console.log(a.intersection(b));
console.log(a.difference(b), a.symmetricDifference(b));
console.log(new Set([3]).isSubsetOf(a), a.isDisjointFrom(new Set([9])));
// the classic way, still common in interviews:
console.log([...a].filter((x) => b.has(x)));

Output

Set(5) { 1, 2, 3, 4, 5 }
Set(2) { 3, 4 }
Set(2) { 1, 2 } Set(3) { 1, 2, 5 }
true true
[ 3, 4 ]

Equality of keys

JavaScript

const s = new Set([NaN, NaN, 0, -0, {}, {}]);
console.log(s.size);             // NaN once, 0/-0 once, two distinct objects
const m = new Map([[[1], "a"]]);
console.log(m.get([1]));         // a different array → different key

Output

4
undefined

When would you use a Map instead of an object?

How do you remove duplicates from an array?