Maps and Sets

Map stores key-value pairs with keys of any type. Set stores unique values. Both preserve insertion order and expose clear size and iteration APIs.

Keyed data with Map

01const progress = new Map();
02progress.set("html", 100);
03progress.set("css", 45);
04
05console.log(progress.get("css"));
06console.log(progress.has("javascript"));
07for (const [course, percent] of progress) console.log(course, percent);

Use a plain object for a record with known property names. Use Map for a changing keyed collection, non-string keys, or frequent additions and removals.

Unique values with Set

01const topics = new Set(["html", "css", "html"]);
02topics.add("javascript");
03const uniqueTopics = [...topics];

Objects are compared by identity, not by matching contents. Two separate { id: 1 } objects are two Set values.

Keep this

Use Map for dynamic keyed collections, Set for uniqueness, and convert deliberately when storing or transmitting them.