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How does a Map differ from a plain object for storing key-value pairs? How do you iterate over a Map's entries? What is a WeakMap and why can't you iterate over its keys? When would you choose a WeakSet over a Set? How do you convert between a Map and a plain object? How would you use a Map to implement an LRU (Least Recently Used) cache? How do WeakRef and FinalizationRegistry extend the weak-reference capabilities beyond WeakMap/WeakSet? Why can't you use WeakMap for caching results keyed by primitive values like strings or numbers? How would you group an array of objects by a property using Map, and how does the newer Object.groupBy() compare?
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JavaScript › Map, Set, WeakMap & WeakSet

How does a Set differ from an array?

Beginner

Answer

A Set only stores unique values — adding a duplicate is a no-op — and lookups with .has() are much faster (O(1) on average) than an array's .includes() (O(n)).
const set = new Set([1, 2, 2, 3]);
console.log(set.size);      // 3
console.log(set.has(2));    // true
Real-world example Tracking which user IDs have already been processed to avoid duplicate work.

Common follow-ups: How do you convert a Set back into an array?

Arrays & Array Methods
All 10 Map, Set, WeakMap & WeakSet questions