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WeakMap

WeakMap

Introduction

WeakMap is a Map variant where keys must be objects and references are weak — if no other references exist to a key object, the entry is garbage collected.

Basic Usage

const wm = new WeakMap();
let user = { name: 'Alice' };

wm.set(user, { role: 'admin' });
console.log(wm.get(user)); // { role: 'admin' }
console.log(wm.has(user)); // true

user = null; // The WeakMap entry is auto-removed!

Limitations

Keys must be objects (no primitives)
No iteration methods (no .keys(), .values(), .forEach())
No .size property
Only .set(), .get(), .has(), .delete() available

Use Cases

// Private data
const privateData = new WeakMap();
class Person {
  constructor(name, ssn) {
    privateData.set(this, { ssn });
  }
  getSSN() { return privateData.get(this).ssn; }
}

// DOM element metadata (auto-cleaned on removal)
const elementData = new WeakMap();
elementData.set(btn, { clicks: 0 });

WeakMap vs Map

Feature	Map	WeakMap
Key types	Any	Objects only
References	Strong	Weak
Iterable	✅	❌
Size	✅	❌

Further Reading