Map, Set, WeakMap & WeakSet

// Map - any key type, ordered
const userRoles = new Map();
userRoles.set("Alice", "admin");
userRoles.set("Bob", "user");
userRoles.set(42, "system");  // number key!

console.log(userRoles.get("Alice")); // "admin"
console.log(userRoles.has("Bob"));   // true
console.log(userRoles.size);         // 3

// Iterate Map
for (const [key, value] of userRoles) {
  console.log(`${key} → ${value}`);
}

// Object as key (impossible with regular objects!)
const user = { name: "Alice" };
const metadata = new Map();
metadata.set(user, { lastLogin: new Date(), visits: 42 });
console.log(metadata.get(user)); // { lastLogin: ..., visits: 42 }

// Set - unique values only
const nums = new Set([1, 2, 3, 2, 1, 4, 3, 5]);
console.log(nums);      // Set { 1, 2, 3, 4, 5 } - no duplicates!
console.log(nums.size);  // 5

nums.add(6);
nums.delete(1);
console.log(nums.has(3)); // true

// Deduplicate array
const arr = [1, 2, 2, 3, 3, 3, 4];
const unique = [...new Set(arr)];
console.log(unique);  // [1, 2, 3, 4]

// Real-world: track unique visitors
const visitors = new Set();
visitors.add("user_123");
visitors.add("user_456");
visitors.add("user_123");  // duplicate - ignored
console.log(`Unique visitors: ${visitors.size}`); // 2