What’s the difference between a Map and a plain Object in JavaScript?

Q: What’s the difference between a Map and a plain Object in JavaScript?

The Core Idea Use Map when you need a true key-value collection with non-string keys and predictable iteration. Use a plain Object when modeling a fixed record with known string keys.

HighEasyJavascript

By FrontendAtlas Team · Updated Jan 31, 2026

Quick Answer

Map is a dedicated key-value collection with any key type, stable iteration order, and a size property. Plain Objects are great for structured records, but keys are limited to strings/symbols and require care with prototypes and ordering.

Answer

The Core Idea

Use Map when you need a true key-value collection with non-string keys and predictable iteration. Use a plain Object when modeling a fixed record with known string keys.

Aspect	Map	Object
Key types	Any type (objects, functions, primitives)	Strings or symbols only (other types coerce)
Iteration order	Guaranteed insertion order	Mostly insertion order, but special cases for numeric keys
Size	`map.size`	No built-in size (use `Object.keys`)
Iteration	Built-in iterators (`map.keys()`, `map.entries()`)	Use `Object.keys/values/entries`
Prototype safety	No prototype collisions	Potential collisions unless created with `Object.create(null)`
Serialization	Not JSON-serializable by default	JSON-friendly for plain data

Key differences between Map and Object

JAVASCRIPT

const map = new Map();
const keyObj = { id: 1 };
map.set(keyObj, 'value');

const obj = {};
obj[keyObj] = 'value';

console.log(map.get(keyObj)); // 'value'
console.log(obj); // { "[object Object]": "value" }

Summary

Map is purpose-built for dynamic key-value data with any key type and stable iteration. Objects are best for structured records and JSON-shaped data.