map VS object

JavaScriptJunior~2 min read

Everyone thinks Map and a regular object {} are almost the same thing, but in reality there are significant differences between them. Let's break it down point by point.

The main idea:

Object is a "general-purpose data structure" for storing an object's properties, whose keys are strings or symbols.
Map is a specialized collection for storing "key -> value" pairs with keys of any type, preserving insertion order.

1. Key type

Feature	`Object`	`Map`
Keys	Only strings or symbols	Any type (strings, numbers, objects, functions, etc.)

Example:

javascript

const obj = {};
const map = new Map();

const keyObj = { id: 1 };

obj[keyObj] = "object value";
map.set(keyObj, "map value");

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

In the object, the key turned into the string "[object Object]", while in Map it stayed a real object.

2. Order preservation

Feature	`Object`	`Map`
Key order	Partially guaranteed (ES6+)	Always preserved

Map always returns elements in insertion order, while an object can behave differently with numeric keys:

javascript

const obj = { 2: "b", 1: "a" };
console.log(Object.keys(obj)); // ["1", "2"] (numbers are sorted)

javascript

const map = new Map([[2, "b"], [1, "a"]]);
console.log([...map.keys()]); // [2, 1] (order is preserved)

3. Easy to get the size

Feature	`Object`	`Map`
Size	`Object.keys(obj).length`	`map.size`

javascript

const map = new Map([["a", 1], ["b", 2]]);
console.log(map.size); // 2

4. Performance

Feature	`Object`	`Map`
Frequent insertions / deletions	Slower	Faster
Iterating over elements	Inconvenient	Convenient via `for...of`

Map is optimized specifically for frequent additions, deletions, and traversal.

5. Iterability

Feature	`Object`	`Map`
Iterates directly	No	Yes

javascript

const map = new Map([["x", 10], ["y", 20]]);
for (const [key, value] of map) {
  console.log(key, value);
}
// x 10
// y 20

For an object, you have to call Object.keys(), Object.entries(), and so on.

6. Prototype and inherited properties

Feature	`Object`	`Map`
Inherits from `Object.prototype`	Yes	No
Can conflict with methods (`toString`, `hasOwnProperty`)	Yes	No

Map has no "clutter" from built-in methods, so any key can be used safely.

7. Converting between them

javascript

const obj = { a: 1, b: 2 };
const map = new Map(Object.entries(obj));

console.log(map); // Map(2) { 'a' => 1, 'b' => 2 }

const backToObj = Object.fromEntries(map);
console.log(backToObj); // { a: 1, b: 2 }

Object.entries() and Object.fromEntries() let you convert easily in both directions.

8. When to use which

Use	When
`Object`	a data structure with fixed properties (for example, a user model)
`Map`	dynamic keys, any key types, a cache, frequent add/remove operations

Summary:

Feature	`Object`	`Map`
Key type	string / symbol	any type
Key order	partially ordered	preserved
Size	must be counted manually	`.size`
Iterability	via `Object.keys()`	directly (`for...of`)
Performance	lower	higher
Use case	static data	dynamic collections

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