Set, Map, WeakSet and WeakMap in JavaScript

JavaScript provides built-in data structures that make working with collections more convenient and efficient. Let's examine Set, Map, WeakSet and WeakMap.

Set

Set is a collection of unique values where each value can appear only once.

Set Features

• Stores only unique values.
• Values can be of any type.
• Unordered structure.

Main Set Methods

• add(value) — adds value.
• delete(value) — removes value.
• has(value) — checks value presence.
• clear() — clears collection.
• size — returns number of elements.

Set Usage Example

const set = new Set();
set.add(1);
set.add(2);
set.add(2); // Ignored, because 2 is already in collection
console.log(set.has(1)); // true
console.log(set.size);   // 2
set.delete(1);
console.log(set);        // Set { 2 }

Map

Map is a collection of key-value pairs where keys can be of any type.

Map Features

• Ordered structure.
• Allows using objects as keys.

Main Map Methods

• set(key, value) — adds key-value pair.
• get(key) — returns value by key.
• delete(key) — removes pair by key.
• has(key) — checks key presence.
• clear() — clears collection.
• size — returns number of elements.

Map Usage Example

const map = new Map();
map.set('name', 'John');
map.set({ id: 1 }, 'Object');
console.log(map.get('name')); // John
console.log(map.size);        // 2
map.delete('name');
console.log(map);             // Map { { id: 1 } => 'Object' }

WeakSet

WeakSet is a collection that stores only objects. These objects are weakly referenced, allowing garbage collector to remove them if there are no other references.

WeakSet Features

• Stores only objects.
• Cannot get size or iterate elements.
• Used for temporary object storage.

Main WeakSet Methods

• add(value) — adds object.
• delete(value) — removes object.
• has(value) — checks object presence.

WeakSet Usage Example

const weakSet = new WeakSet();
let obj = { name: 'John' };
weakSet.add(obj);
console.log(weakSet.has(obj)); // true
obj = null; // Object will be removed from WeakSet by garbage collector

WeakMap

WeakMap is a collection of key-value pairs where keys can only be objects and values can be anything. Keys are weakly referenced, allowing garbage collector to remove objects if there are no more references.

WeakMap Features

• Keys can only be objects.
• Values can be anything.
• Elements are weakly referenced, meaning automatic removal from collection if key is unreachable.
• Cannot iterate collection elements or get its size.

Main WeakMap Methods

• set(key, value) — adds key-value pair.
• get(key) — returns value by key.
• delete(key) — removes key-value pair.
• has(key) — checks key presence.

WeakMap Usage Example

const weakMap = new WeakMap();

// Create object
let user = { name: "John" };

// Add object as key and value
weakMap.set(user, "User data");

console.log(weakMap.get(user)); // "User data"

// Remove reference to object
user = null;

// Object will be removed from WeakMap by garbage collector