Module 33 // Core JavaScript

Map, Set, WeakMap, WeakSet

Module Objective

Key equality, weak references, garbage collection behavior

Mental Model Realtime Simulation

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Practical Code Examples

// Example 1
// 1. Removing Duplicates
const raw = [1, 5, 2, 1, 5, 3];
const unique = [...new Set(raw)];

console.log(unique); // [1, 5, 2, 3]
💡 Converting an array to a `Set` and back to an array is the fastest way to remove duplicates in modern JavaScript.
// Example 2
// 2. Objects as Map Keys
const user = { name: "Subhajit" };
const metadata = new Map();

metadata.set(user, { lastLogin: "2025-12-17" });

console.log(metadata.get(user)); // { lastLogin: ... }
💡 In a regular Object, keys are converted to strings. A `Map` allows you to use actual objects as keys, which is powerful for associating metadata without modifying the original object.
// Example 3
// 3. WeakMap for Private Data
const privates = new WeakMap();

class User {
  constructor(id) {
    privates.set(this, { id });
  }
  getId() {
    return privates.get(this).id;
  }
}

const me = new User(42);
console.log(me.getId()); // 42
💡 `WeakMap` doesn't prevent its keys from being **Garbage Collected**. If the `User` instance is deleted, the private data in the `WeakMap` is automatically removed from memory.

Engine & Memory Architecture

Hash Table Internals

1. Hash Maps:

  • Map and Set use Hash Tables in the Heap for O(1) average time complexity for access.
  • Standard Objects also use hash tables, but Map is optimized for frequent additions/removals.

2. Garbage Collection (Weak Variants):

  • Strong Reference: A standard Map keeps its keys alive in the RAM as long as the map exists.
  • Weak Reference: A WeakMap holds a "weak" reference. If no other code points to the key object, the Garbage Collector can reclaim that memory even if it's still in the map.

3. Memory Overhead:

  • Each Map entry requires more memory than a standard Object property due to the complexity of the Hash Table structure.
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