Generator ও Iterator

const numbers = [10, 20, 30]
const iterator = numbers[Symbol.iterator]()

iterator.next()   // { value: 10, done: false }
iterator.next()   // { value: 20, done: false }
iterator.next()   // { value: 30, done: false }
iterator.next()   // { value: undefined, done: true }

function* countUpTo(max) {
  let count = 1
  while (count <= max) {
    yield count
    count++
  }
}

const counter = countUpTo(3)
counter.next()   // { value: 1, done: false }
counter.next()   // { value: 2, done: false }
counter.next()   // { value: 3, done: false }
counter.next()   // { value: undefined, done: true }

for (const n of countUpTo(3)) {
  console.log(n)   // 1, 2, 3
}

[...countUpTo(3)]   // [1, 2, 3]

function* infiniteIds() {
  let id = 1
  while (true) {
    yield id++
  }
}

const ids = infiniteIds()
ids.next().value   // 1
ids.next().value   // 2
// আসলে কখনো একটি infinite array বানায় না — শুধু একবারে একটা মান তৈরি করে