What are rest parameters and the spread operator? Rest parameters और spread operator क्या हैं?

Rest parameters (...) allow you to collect multiple arguments into an array. The spread operator (...) does the opposite - it expands an array or object into individual elements.

// Rest parameters - collect into array
function sum(...numbers) {
    return numbers.reduce((a, b) => a + b, 0);
}
console.log(sum(1, 2, 3, 4, 5)); // 15

// Rest with named parameters
function greet(greeting, ...names) {
    console.log(greeting + ' ' + names.join(', '));
}
greet('Hello', 'John', 'Jane', 'Bob'); // 'Hello John, Jane, Bob'

// Spread with arrays
const arr1 = [1, 2, 3];
const arr2 = [...arr1, 4, 5]; // [1, 2, 3, 4, 5]

const max = Math.max(...[1, 5, 3, 2]); // equivalent to Math.max(1, 5, 3, 2)
console.log(max); // 5

// Spread with objects
const user1 = { name: 'John', age: 30 };
const user2 = { ...user1, email: 'john@gmail.com' };
// { name: 'John', age: 30, email: 'john@gmail.com' }

// Spread to merge objects
const obj1 = { a: 1, b: 2 };
const obj2 = { b: 3, c: 4 }; // b will be overwritten
const merged = { ...obj1, ...obj2 }; // { a: 1, b: 3, c: 4 }

// Rest in arrow functions
const multiply = (...nums) => nums.reduce((a, b) => a * b, 1);
console.log(multiply(2, 3, 4)); // 24

// Difference: Rest collects, Spread expands
const nums = [1, 2, 3];

// Spread: expand array
function test(a, b, c) {
    console.log(a, b, c);
}
test(...nums); // 1, 2, 3

// Rest: collect arguments
function collect(...args) {
    console.log(args); // [1, 2, 3]
}
collect(1, 2, 3);

Interview tip: Rest parameters must be the last parameter in the function signature. Spread operator can be used with arrays, objects, function calls, and function parameters. They're ES6 features that enable more flexible code.