What are WeakMap and WeakSet used for in JavaScript?

Question

What are WeakMap and WeakSet used for in JavaScript?

Answer

WeakMap and WeakSet store weak references to objects — meaning if there are no other references to that object, it can be garbage collected.

• WeakMap → key-value pairs where keys must be objects
• WeakSet → set of objects, no primitives

const wm = new WeakMap();
let obj = {};
wm.set(obj, 'private data');
obj = null; // object can now be garbage collected

This is useful for caching, storing metadata, or private data tied to object lifecycles.

Real-World Example

Libraries use WeakMap to store component metadata without risking memory leaks.

Quick Practice

Create a WeakMap and store a DOM element as a key with some data. Then remove the DOM element — explain what happens.

Summary

Use WeakMap/WeakSet when you want to associate data with objects without preventing garbage collection.