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What makes an object 'iterable' in JavaScript? Can you use a for...of loop directly on a generator's return value? How does yield pause and resume execution, and what does it return to the caller? How do you delegate from one generator to another using yield*? How do you implement an infinite sequence using a generator, and why is this safe? How do you use generator.throw() and generator.return() to control a running generator externally? How were generators historically used to write asynchronous code before async/await existed? How would you implement a custom Symbol.iterator for a tree data structure to enable depth-first traversal with for...of? What's the performance and memory advantage of a generator-based lazy pipeline over building intermediate arrays with map/filter?
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JavaScript › Iterators & Generators

How do you write a basic generator function?

Beginner

Answer

Add an asterisk after 'function' to make it a generator; calling it doesn't run the body immediately but returns an iterator, and each yield pauses execution, resuming from that point on the next next() call.
function* countTo3() {
  yield 1;
  yield 2;
  yield 3;
}
const gen = countTo3();
console.log(gen.next().value); // 1
console.log(gen.next().value); // 2
Real-world example Lazily generating a sequence of values one at a time instead of computing and storing them all upfront.

Common follow-ups: What does gen.next() return once the generator is finished?

Types & Coercion
All 10 Iterators & Generators questions