How do you remove duplicates from an array?
Quick answer
Wrap it in a Set and spread back: [...new Set(arr)].
In detail
A Set stores unique values, so converting to a Set and back removes duplicates in one line for primitives. For objects (compared by reference) you need a key-based approach with a Map or filter.
[...new Set([1, 2, 2, 3])]; // [1, 2, 3]
// by key for objects:
const seen = new Set();
users.filter(u => !seen.has(u.id) && seen.add(u.id));Why interviewers ask this: A quick practical that shows knowledge of Set.
This is 1 of 118+ questions in the JavaScript Interview Kit
Get every question with detailed answers, follow-ups and real code — plus coding challenges and a last-minute revision sheet. One-time payment, instant access.
⚡ Get the JavaScript Interview Kit → ₹299