Map and Set

A Set holds unique values: adding a duplicate does nothing, and .has() checks membership quickly. A Map stores key-value pairs like an object, but its keys can be any type, it remembers insertion order, and it has a .size. Use .set, .get, .has and .delete, and loop with for (const [key, value] of map).

How it reads

• The duplicate "vega" is ignored, so the set has 3 items
• visits.get("Mars") + 1 reads, updates and writes back a count
• [...new Set(array)] is the quickest way to remove duplicates

Cloud tip: Reach for a Map when keys are added and removed often, or are not strings. Plain objects are fine for fixed shapes.