Map

The Map object is a collection of key-value pairs where keys can be of any type (including objects, functions, and primitives). Unlike plain objects, Maps preserve insertion order and provide better performance for frequent additions and deletions.

What Problem Does Map Solve?

Traditional objects have limitations:

• Keys are always converted to strings
• Keys must be unique but type coercion causes issues
• No built-in size property
• Iteration order is not guaranteed
• Prototype inheritance can cause collisions

Key Characteristics

Feature	Description
Key types	Any value (objects, functions, primitives)
Order	Maintains insertion order
Size	.size property for quick length check
Performance	Optimized for frequent additions/deletions
Iteration	Directly iterable with for...of

Syntax

const map = new Map();
const mapFromIterable = new Map([
  ["key1", "value1"],
  ["key2", "value2"],
]);

Key Points

• Maps can use any value as a key (objects, functions, NaN).
• Keys are compared using the SameValueZero algorithm.
• Insertion order is preserved during iteration.
• Maps are iterable by default (returns [key, value] pairs).
• NaN can be used as a key (only one NaN key).

Example Code

// Creating a new Map
const map = new Map();

// Using objects as keys
const user1 = { id: 1 };
const user2 = { id: 2 };

map.set(user1, "Alice");
map.set(user2, "Bob");

console.log(map.get(user1)); // 'Alice'
console.log(map.get(user2)); // 'Bob'

// Using other primitives as keys
map.set(42, "The answer");
map.set(true, "Boolean key");
map.set("name", "String key");
map.set(NaN, "Not a number");

console.log(map.get(42)); // 'The answer'
console.log(map.get(true)); // 'Boolean key'
console.log(map.get(NaN)); // 'Not a number'

// Creating a Map from an iterable
const fromArray = new Map([
  ["a", 1],
  ["b", 2],
  ["c", 3],
]);

console.log(fromArray.get("a")); // 1
console.log(fromArray.get("b")); // 2
console.log(fromArray.get("c")); // 3

// Map size
console.log(map.size); // Number of entries