What are generators and iterators in JavaScript?

Question

What are generators and iterators in JavaScript?

Answer

An iterator is an object that defines a sequence and returns a value on each call to next().

const iterator = [1, 2, 3][Symbol.iterator]();
console.log(iterator.next()); // { value: 1, done: false }

A generator is a special function (declared with function*) that makes writing iterators easier.

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

const gen = idGenerator();
console.log(gen.next().value); // 1
console.log(gen.next().value); // 2

Generators:

• Pause and resume execution
• Are great for producing sequences
• Power async flows (with older libraries)

Real-World Example

You can use generators to lazily load data or paginate large lists without loading everything at once.

Quick Practice

Create a generator that yields Fibonacci numbers one by one.

Summary

Iterators define “how to loop”. Generators make building iterators simple and readable.