WeakMap and WeakSet

Syntax

const wm = new WeakMap();
wm.set(objectKey, value);

Examples

const privateData = new WeakMap();

class User {
  constructor(name) {
    privateData.set(this, { loginAttempts: 0 });
    this.name = name;
  }

  recordLoginAttempt() {
    const data = privateData.get(this);
    data.loginAttempts++;
    return data.loginAttempts;
  }
}

const user = new User("Alice");
console.log(user.recordLoginAttempt()); // 1
// If "user" is later garbage collected, its entry in privateData is automatically cleaned up too

const processedItems = new WeakSet();

function processItem(item) {
  if (processedItems.has(item)) {
    console.log("Already processed, skipping");
    return;
  }
  processedItems.add(item);
  console.log("Processing item...");
}

const obj = { id: 1 };
processItem(obj); // "Processing item..."
processItem(obj); // "Already processed, skipping"

Best practices

• Use WeakMap when attaching metadata to objects you do not own or control the lifecycle of, so it does not prevent garbage collection
• Remember WeakMap/WeakSet are not iterable and have no size property - they intentionally hide their contents since entries can disappear at any time via garbage collection
• Only use object references as keys (WeakMap) or values (WeakSet) - primitives like strings or numbers are not allowed
• Reach for a regular Map when you need to iterate over entries or know how many there are - WeakMap is specifically for the memory-safe metadata use case

At a glance

Purpose

Application logic and interaction

File extension

.js · .mjs

Runs in

Browsers and JavaScript runtimes

Usually used with

HTML, CSS and Web APIs

Specifications & further reading

• ECMAScript specification

Related JavaScript documentation