Map and Set in JavaScript

Source: Dev.to

What Is a Map?

A Map is a collection of key‑value pairs, similar to an object, but with several improvements:

• Keys can be any type (objects, functions, primitives)
• Preserves insertion order
• Built‑in methods to get, set, delete, and check entries

Map Example

const map = new Map();

map.set("name", "Rahul");
map.set("age", 22);
map.set(true, "Boolean key");

console.log(map.get("name")); // Rahul
console.log(map.has("age"));  // true
console.log(map.size);        // 3

Map Visual

Key → Value
"name"  → "Rahul"
"age"   → 22
true    → "Boolean key"

Map vs Object

What Is a Set?

A Set is a collection of unique values—duplicates are automatically removed.

• Can store any type of value
• Provides fast lookups and uniqueness checks

Set Example

const set = new Set([1, 2, 3, 2, 1]);

console.log(set);          // Set(3) { 1, 2, 3 }
console.log(set.has(2));   // true

set.add(4);
console.log(set);          // Set(4) { 1, 2, 3, 4 }

Set Visual

[1, 2, 3, 2, 1] → Set → {1, 2, 3}

Set vs Array

When to Use Map and Set

Use Map when:

• You need dynamic keys of any type
• You want ordered key‑value storage
• You need built‑in iteration over entries

Use Set when:

• You want unique values only
• You need to quickly check existence
• You want to remove duplicates from an array

Removing Duplicates Example

const numbers = [1, 2, 3, 2, 4, 3];
const uniqueNumbers = [...new Set(numbers)];

console.log(uniqueNumbers); // [1, 2, 3, 4]

Map for Counting Occurrences

const arr = ["apple", "banana", "apple"];
const countMap = new Map();

arr.forEach(item => {
  countMap.set(item, (countMap.get(item) || 0) + 1);
});

console.log(countMap);
// Map(2) { "apple" => 2, "banana" => 1 }

Key Takeaways

• Map → flexible key‑value storage, ordered, keys can be any type
• Set → collection of unique values, fast lookups, removes duplicates easily
• Both solve common limitations of objects and arrays.