Rest ও Spread Operator

function sum(...numbers) {
  return numbers.reduce((total, n) => total + n, 0)
}

sum(1, 2, 3)        // 6
sum(1, 2, 3, 4, 5)  // 15

const [first, ...rest] = [10, 20, 30, 40]
first   // 10
rest    // [20, 30, 40]

const { name, ...otherFields } = { name: 'Sam', age: 28, city: 'Delhi' }
otherFields   // { age: 28, city: 'Delhi' }

const a = [1, 2, 3]
const b = [4, 5, 6]
const combined = [...a, ...b]
// [1, 2, 3, 4, 5, 6]

Math.max(...a)   // 3 — array-কে আলাদা argument-এ spread করে

const defaults = { theme: 'light', fontSize: 16 }
const userPrefs = { fontSize: 20 }

const settings = { ...defaults, ...userPrefs }
// { theme: 'light', fontSize: 20 } — পরের key আগেরটাকে override করে