What is a JavaScript generator function?

A JavaScript generator function is defined as a function that can be paused and resumed, which is fundamentally what makes it distinct from regular functions. When a generator function is invoked, it does not execute its code immediately. Instead, it returns a generator object, which can be used to control the execution of the function.

The key characteristic of a generator function is the use of the yield keyword. Each time the generator's next() method is called, the function executes until it encounters a yield statement. At that point, it pauses and returns the value specified by yield, but it preserves the function's state. The next time next() is invoked, execution resumes from where it was paused, allowing for an iterative process that can produce a sequence of values over time.

This functionality makes generator functions particularly useful in situations where you want to produce a sequence of values on the fly, maintain state between successive iterations, or handle asynchronous programming in a more manageable way.

Other options describe different concepts or capabilities that do not accurately characterize generator functions. For example, while it's possible for a generator to produce Fibonacci numbers, this alone does not define a generator function. Additionally, while a generator can return multiple values over time, it does