JavaScript Map vs WeakMap

Map

A Map is a collection of key-value pairs.

Characteristics

• Keys can be of any type:
  • Object
  • String
  • Number
  • Boolean
  • Symbol
  • Function
• Values can be of any type.
• Maintains strong references to its keys.
• Iterable.
• Has a size property.
• Supports methods like:
  • set()
  • get()
  • has()
  • delete()
  • clear()

Example

const map = new Map();

const user = { name: "Chandan" };

map.set(user, "Premium");

console.log(map.get(user)); // Premium

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WeakMap

A WeakMap is a special type of Map where:

• Keys must be objects.
• Keys are held weakly, allowing garbage collection when no other references exist.
• Values can be of any type.

Example

const weakMap = new WeakMap();

let user = { name: "Chandan" };

weakMap.set(user, "Premium");

console.log(weakMap.get(user)); // Premium

user = null;
// The entry becomes eligible for garbage collection.

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What Does "Weak" Mean?

Map

let obj = { x: 10 };

const map = new Map();
map.set(obj, "data");

obj = null;

The object is not garbage collected because Map still holds a strong reference.

Map
 └── { x: 10 } → "data"

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WeakMap

let obj = { x: 10 };

const weakMap = new WeakMap();
weakMap.set(obj, "data");

obj = null;

After obj becomes null, there are no strong references to the object.

The JavaScript garbage collector can remove:

• The object
• Its corresponding WeakMap entry

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Why Is WeakMap Not Iterable?

Since entries can disappear at any time due to garbage collection, iteration would be unreliable.

Therefore, WeakMap does not support:

• for...of
• keys()
• values()
• entries()
• size

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Common Use Case

Store private metadata associated with objects.

const privateData = new WeakMap();

function createUser(user) {
    privateData.set(user, {
        loginCount: 0
    });
}

function login(user) {
    privateData.get(user).loginCount++;
}

When the user object is no longer referenced elsewhere, its associated data is automatically removed.

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Map vs. WeakMap

Feature	Map	WeakMap
Key types	Any value	Objects only
Value types	Any value	Any value
Strong reference to keys	Yes	No
Garbage collection	Keys remain until removed	Entries removed automatically when keys become unreachable
Iterable	Yes	No
size property	Yes	No
clear()	Yes	No
Best use case	General-purpose key-value storage	Private metadata and avoiding memory leaks