Generators

A generator function, written function*, can pause itself with yield and pick up again later. Calling it does not run the body; it returns a generator object. Each next() runs until the next yield and returns { value, done }. Generators produce values lazily, so they can even describe endless sequences, and they work directly with for...of and spread.

How it reads

• yield hands out one value and pauses the function right there
• Spreading [...countdown(3)] pulls values until the generator finishes
• The endless ids generator is safe because values are only made on demand

Cloud tip: Never spread an endless generator. Take what you need with next() or a loop with a break.