Lesson 83 - WeakMaps in JavaScript

What is a WeakMap?

A WeakMap is similar to a Map but with some differences that make it more suitable for storing data tied to object lifecycles. Keys in a WeakMap must be objects, and they are held "weakly," meaning if there are no other references to the object, it can be garbage collected.

Key Differences from Map

Creating a WeakMap

const weakMap = new WeakMap();
let user = { name: "Jane" };
weakMap.set(user, "Active");
console.log(weakMap.get(user)); // Active

Note: If the user object is no longer referenced elsewhere, it will be garbage collected, and the WeakMap entry will disappear automatically.

Valid Usage

const wm = new WeakMap();

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

wm.set(obj1, "Data for obj1");
wm.set(obj2, "Data for obj2");

console.log(wm.has(obj1)); // true
wm.delete(obj2);           // removes obj2 from the WeakMap

Invalid Usage

const wm = new WeakMap();
wm.set("name", "Lance"); //  Error: keys must be objects

Use Cases

• Private data for class instances
• Caching or memoization tied to object references
• Event listeners or DOM nodes storage without memory leaks

Try Your Hand

1. Create a WeakMap and store some metadata related to a user object.
2. Use get() to retrieve the data.
3. Check if the key exists using has().
4. Delete the key from the WeakMap.