Interview question

What are Map and Set in JavaScript, and how do they differ from plain objects and arrays? JavaScript में Map और Set क्या हैं, और plain objects/arrays से कैसे अलग हैं?

Answer

Structure	Key type	Order preserved?	Size property
Object	String/Symbol only	Mostly, with edge cases	No direct property, need Object.keys().length
Map	Any type (objects, functions, etc.)	Yes, insertion order	.size
Array	Numeric index only	Yes	.length
Set	N/A - stores unique values	Yes, insertion order	.size

// MAP - key-value pairs with ANY key type
const userRoles = new Map();
const userObj = { id: 1 };

userRoles.set('john', 'admin');
userRoles.set(userObj, 'editor'); // object as a key - impossible with plain objects

console.log(userRoles.get('john')); // admin
console.log(userRoles.get(userObj)); // editor
console.log(userRoles.size); // 2

// Iterating a Map
for (const [key, value] of userRoles) {
    console.log(key, value);
}

userRoles.has('john'); // true
userRoles.delete('john');

// Converting Map to array
const entries = [...userRoles]; // [[key, value], ...]

// SET - collection of unique values
const uniqueNumbers = new Set([1, 2, 2, 3, 3, 3]);
console.log(uniqueNumbers); // Set { 1, 2, 3 } - duplicates removed
console.log(uniqueNumbers.size); // 3

uniqueNumbers.add(4);
uniqueNumbers.has(2); // true
uniqueNumbers.delete(1);

// Practical use: removing duplicates from an array
const arr = [1, 2, 2, 3, 3, 4];
const unique = [...new Set(arr)];
console.log(unique); // [1, 2, 3, 4]

// Iterating a Set
for (const value of uniqueNumbers) {
    console.log(value);
}

Structure	Key type	Order?	Size
Object	सिर्फ String/Symbol	ज़्यादातर	Object.keys().length
Map	कोई भी type	हाँ, insertion order	.size
Set	Unique values	हाँ	.size

// MAP - key-value pairs, कोई भी key type
const userRoles = new Map();
const userObj = { id: 1 };

userRoles.set('john', 'admin');
userRoles.set(userObj, 'editor'); // object key - plain object में impossible

console.log(userRoles.get('john')); // admin
console.log(userRoles.size); // 2

// Map iterate करना
for (const [key, value] of userRoles) {
    console.log(key, value);
}

userRoles.has('john'); // true
userRoles.delete('john');

// SET - unique values का collection
const uniqueNumbers = new Set([1, 2, 2, 3, 3, 3]);
console.log(uniqueNumbers); // Set { 1, 2, 3 }
console.log(uniqueNumbers.size); // 3

uniqueNumbers.add(4);
uniqueNumbers.has(2); // true

// Array से duplicates हटाना
const arr = [1, 2, 2, 3, 3, 4];
const unique = [...new Set(arr)];
console.log(unique); // [1, 2, 3, 4]

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