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

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