Data Structureseasy

What is a Set and when would you use it?

Answer

A Set stores unique values (no duplicates). Use it for fast membership checks and deduplication (e.g., keep unique user IDs).

const ids = new Set<number>();
ids.add(42);
console.log(ids.has(42)); // true

Advanced answer

Deep dive

A Set is an abstract data type built around one idea: store unique values and answer membership queries quickly (have I seen X?). In many languages it is backed by a hash table, which gives average O(1) add/has/delete, but can degrade to O(n) in worst cases (poor hashing, adversarial input, very high load factor).

Use a Set when:

You need deduplication (unique IDs, unique tags, unique emails).
You need fast membership checks (feature flags, allowed roles, cache keys).
You track visited nodes/states in traversals (BFS/DFS/Dijkstra).

Examples

const users = ['a', 'b', 'a', 'c']
const unique = [...new Set(users)] // ['a', 'b', 'c']

const allowed = new Set(['admin', 'editor'])
const canEdit = allowed.has(role)

Language notes

Java: HashSet uses hashCode() + equals(). If you store custom objects, implement both consistently.
JS/TS: Set compares primitives by value, but objects by identity (two objects with same fields are still different).

Common pitfalls

Expecting Set to deduplicate objects by fields (it will not).