What are generator functions, and how do yield and next() work?

Direct answer

A generator function declared with function* returns a generator object; execution can pause at yield and later resume when next() is called, making generators useful for lazy iteration and controllable sequences.

Calling a generator function does not execute its body to completion immediately. It returns a generator object. The first next() begins execution until a yield (or return/end), producing an iterator result.

For function* ids(){ yield 1; yield 2; }, repeated next() calls produce values 1 and 2, then a result with done: true.

Generators work naturally with for...of and spread because generator objects implement the iteration protocols.
A value passed to a later next(value) can become the result of the suspended yield expression inside the generator.
return() and throw() allow consumers to influence generator completion/error flow.
Use generators when laziness or pull-based iteration clarifies the model; do not replace simple array transformations with generators purely for cleverness.