Generators Pattern

The ES2015 specification introduced a syntax construct that’s closely related to iterators. We’re talking about generators, also known as semicoroutines. They’re a generalization of standard functions, in which there can be different entry points. In a standard function, we can have only one entry point, which corresponds to the invocation of the function itself, but a generator can be suspended (using the yield statement) and then resumed at a later time. Among other things, generators are very well suited to implement iterators, in fact, as we’ll see in a bit, the generator object returned by a generator function is indeed both an iterator and an iterable.

Generators in theory

To define a generator function, we need to use the function* declaration (the function keyword followed by an asterisk).

Invoking a generator function won’t execute its body immediately. Rather, it’ll return a generator object, which, as we already mentioned, is both an iterator and an iterable. But it doesn’t end here; invoking next() on the generator object will start or resume the execution of the generator until the yield instruction is invoked or the generator returns (either implicitly or explicitly with a return instruction). Within the generator, using the keyword yield followed by a value x is equivalent to returning {done: false, value: x} from the iterator, while returning a value x is ...