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More in Map, Set, WeakMap & WeakSet
How does a Set differ from an array? 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 Map differ from a plain object for storing key-value pairs?

Beginner

Answer

A Map allows keys of ANY type (objects, functions, even NaN), preserves insertion order reliably, has a .size property, and doesn't come with inherited prototype properties that could collide with your keys — plain objects only reliably support string/Symbol keys.
const map = new Map();
map.set('name', 'Sam');
map.set(42, 'answer');
console.log(map.get(42)); // 'answer'
console.log(map.size);   // 2
Real-world example Using an object instance as a key to associate metadata with it, which a plain object can't do.

Common follow-ups: Why might using an object as a plain-object key silently fail (via string coercion)?

Objects Property Descriptors & Immutability
All 10 Map, Set, WeakMap & WeakSet questions