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Plain objects ARE often used as maps in JavaScript, but the built-in Map and Set (ES2015) are better for most cases. They preserve insertion order, accept any key type, and have a clean iteration API.

Set — unique values

From an array (with dedup):

This is the canonical JS dedup pattern.

Iterate Set:

Map — key/value pairs (any key type)

With initial entries:

Why prefer Map over plain object?

Map keys don't collide with prototype properties. Map preserves insertion order reliably across all JS engines. Map can have non-string keys.

Iterate Map:

When to use which

Iteration protocols (recap)

Anything that has [Symbol.iterator]() is iterable:

• Strings
• Arrays
• Sets
• Maps
• Generators
• TypedArrays
• NodeList (DOM)
• arguments

Iterables work with:

• for...of
• Spread [...iter], {...iter} (limited)
• Destructuring [a, b] = iter
• Array.from(iter)
• Promise.all, Set and Map constructors

Plain objects are NOT iterable by default. Use Object.entries(obj), Object.keys(obj), Object.values(obj) to get an array.

Common mistakes

• Plain object as a map with arbitrary string keys — collisions with __proto__, toString, constructor. Use Map or Object.create(null).
• JSON.stringify on a Map — produces {}. Convert to a plain object first or use a custom replacer.
• Set/Map for primitive lookups when you'd be fine with an object — slight overhead. For tiny fixed lists, plain object/array can be fine.
• Comparing Sets with === — references. new Set([1]) === new Set([1]) is false. Compare contents manually.
• Forgetting Sets dedupe by SameValueZero — Set treats NaN as equal to NaN (unlike ===), but objects are by reference.