What are JavaScript Generators and Iterators?

Generators are functions that can pause and resume execution, producing a sequence of values.

Generator Function:

function* numberGenerator() {
  yield 1;
  yield 2;
  yield 3;
  return "done";
}
const gen = numberGenerator();
console.log(gen.next()); // { value: 1, done: false }
console.log(gen.next()); // { value: 2, done: false }
console.log(gen.next()); // { value: 3, done: false }
console.log(gen.next()); // { value: "done", done: true }

Infinite Sequences:

function* fibonacci() {
  let a = 0, b = 1;
  while (true) {
    yield a;
    [a, b] = [b, a + b];
  }
}
const fib = fibonacci();
console.log(fib.next().value); // 0
console.log(fib.next().value); // 1
console.log(fib.next().value); // 1

Iterator Protocol:

const iterable = {
  [Symbol.iterator]() {
    let count = 0;
    return {
      next() {
        if (count < 3) {
          return { value: count++, done: false };
        }
        return { done: true };
      }
    };
  }
};
for (const value of iterable) {
  console.log(value); // 0, 1, 2
}

Use Cases:
- Lazy evaluation
- Memory-efficient sequences
- Async iteration
- State machines
- Custom iterables