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🧭 JavaScript Collections — Map, Set, WeakMap & WeakSet

JavaScript provides built-in collection types for storing groups of values:

  • Map
  • Set
  • WeakMap
  • WeakSet

These offer better performance, flexibility, and functionality than regular objects and arrays for certain use cases.

🔹 1. Map

✅ What is a Map?

A Map is a collection of key-value pairs, similar to objects, but with more features.

🧠 Key Features:

  • Keys can be of any data type (including objects, functions, etc.)
  • Maintains the insertion order
  • Can easily get the size

📦 Syntax:

const myMap = new Map();
myMap.set("name", "Varun");
myMap.set(1, "One");
myMap.set(true, "Boolean");

console.log(myMap.get("name"));  // "Varun"
console.log(myMap.size);         // 3

🔄 Iterating:

for (let [key, value] of myMap) {
    console.log(key, value);
}

🔹 2. Set

✅ What is a Set?

A Set is a collection of unique values.

🧠 Key Features:

  • Stores only unique values
  • Can hold values of any type
  • Maintains insertion order

📦 Syntax:

const mySet = new Set();
mySet.add(1);
mySet.add(2);
mySet.add(2); // ignored, already exists

console.log(mySet.has(1));   // true
console.log(mySet.size);     // 2

🔄 Iterating:

mySet.forEach(value => console.log(value));

🔸 3. WeakMap

✅ What is a WeakMap?

A WeakMap is like a Map, but:

  • Keys must be objects
  • Keys are weakly referenced (not prevented from garbage collection)

🧠 Key Features:

  • Can't iterate or check size
  • Great for storing private data tied to DOM elements or objects
  • Helps avoid memory leaks

📦 Syntax:

const wm = new WeakMap();
let obj = {};

wm.set(obj, "private value");
console.log(wm.get(obj)); // "private value"

obj = null; // Now the key-value pair is eligible for garbage collection

⚠️ WeakMap methods: set, get, has, delete

🔸 4. WeakSet

✅ What is a WeakSet?

A WeakSet is like a Set, but:

  • Only stores objects
  • Objects are held weakly
  • Not iterable

🧠 Key Features:

  • Cannot check size or iterate
  • Useful for managing object references without memory leaks

📦 Syntax:

const ws = new WeakSet();
let user = { name: "Varun" };

ws.add(user);
console.log(ws.has(user)); // true

user = null; // Automatically removed from the WeakSet

🔁 Comparison Table

Feature Map Set WeakMap WeakSet
Keys/Values key-value values key-value values
Key type Any Any Object only Object only
Duplicates No No No No
Order Preserved Preserved N/A N/A
Iterable ✅ ✅ ❌ ❌
Garbage Col. ❌ ❌ ✅ ✅
Size Access ✅ ✅ ❌ ❌

🚀 Use Cases

Use Case Best Collection
Unique values Set
Key-value store with non-string keys Map
Memory-safe object-key storage WeakMap
Track object references WeakSet

🧪 Quick Practice

  • Store usernames uniquely → Set
  • Cache user metadata → Map
  • Track element-specific data safely → WeakMap
  • Track whether objects were seen → WeakSet

🧠 Summary

  • Use Map/Set when you need iteration and full data control
  • Use WeakMap/WeakSet when dealing with temporary object references and memory-sensitive operations

Happy Coding! 🚀