WeakMap in JavaScript

A WeakMap is a collection of key-value pairs where:

• Keys must be objects (no primitives like strings or numbers).
• Values can be any type.
• Keys are weakly referenced, meaning if there are no other references to the key object, it can be garbage-collected.
• Not iterable — you cannot loop over WeakMap entries.
• No .size property or methods to list keys/values.

WeakMap is useful for storing private data or temporary metadata associated with objects.

1. Creating a WeakMap

const weakMap = new WeakMap();

const obj1 = {};
const obj2 = {};

weakMap.set(obj1, "Alice");
weakMap.set(obj2, "Bob");

console.log(weakMap.has(obj1)); // true
console.log(weakMap.get(obj2)); // "Bob"

2. WeakMap Methods

A WeakMap has four methods for managing key-value pairs (keys must be objects, held weakly):

a. set(key, value)

Adds or updates a key-value pair.

const weakMap = new WeakMap();
const user = { name: "John" };

weakMap.set(user, { age: 30 });

b. get(key)

Returns the value associated with the key, or undefined if not present.

console.log(weakMap.get(user)); // { age: 30 }

c. has(key)

Checks if a key exists in the WeakMap.

console.log(weakMap.has(user)); // true

d. delete(key)

Removes a key-value pair. Returns true if the key existed and was removed.

weakMap.delete(user);
console.log(weakMap.has(user)); // false

3. Key Characteristics

Feature	WeakMap
Keys	Objects only
Values	Any type
Reference Type	Weak reference for keys (GC)
Iterable	No
Size property	No
Garbage Collection	Automatic for keys with no references

4. Practical Use Cases

a. Private Object Data

• Private data is not accessible outside the class.
• WeakMap allows automatic cleanup when john is garbage-collected.

const privateData = new WeakMap();

class User {
  constructor(name) {
    privateData.set(this, { secret: `Password for ${name}` });
  }

  getSecret() {
    return privateData.get(this).secret;
  }
}

const john = new User("John");
console.log(john.getSecret()); // Password for John

b. DOM Metadata Tracking

The data is automatically removed if div is removed from the DOM and no other references exist.

const elementData = new WeakMap();

const div = document.createElement("div");
elementData.set(div, { clicked: false });

div.addEventListener("click", () => {
  elementData.get(div).clicked = true;
});

5. Limitations

• Cannot iterate (no for...of or .keys()).
• Cannot get size of the WeakMap.
• Only object keys are allowed.
• Not suitable if you need to enumerate entries.

6. Comparison with Map

Feature	Map	WeakMap
Key Types	Any type	Objects only
Garbage Collection	Strong references	Weak references (GC)
Iterable	Yes	No
Size	.size available	Not available
Use Case	General-purpose	Private/temporary data