Closing Javascript Generators

In this previous post about closing Python generators I mentioned that JavaScript generators had a similar feature that would deserve a post on its own, so here it is.

JavaScript generators have a return() method. We can think of it as partially equivalent to Python's close() method. This is so for the simple (and main I guess) use cases that I explained in my previous post (use it as a replacement for break, and when you are passing the generator around to other methods and one of them can decide to close it). For example:

function* citiesGen() {
    yield "Paris";
    yield "Porto";
    return "Europe";
}

// using .return() rather than break
let cities = citiesGen();
for (let city of cities) {
    if (city == "Porto") {
        cities.return();
        console.log("closing generator");
    }
    console.log(city);
}
/*
Paris
closing generator
Porto
*/

Then we have the more advanced cases, for which indeed finding a use case seems not so apparent to me. Here it's where the differences with Python's close() are important. JavaScript return() accepts a value, that will be returned as part of the value-done pair returned when the generator is finished. This "when it's finished" is key, as a try-finally in the generator code can prevent the return() call from finishing the generator in that call. It will continue to produce values as instructed from the finally part, and once completed will return the value that we had passed in the return() call. The theory:

The return() method, when called, can be seen as if a return value; statement is inserted in the generator's body at the current suspended position, where value is the value passed to the return() method. Therefore, in a typical flow, calling return(value) will return { done: true, value: value }. However, if the yield expression is wrapped in a try...finally block, the control flow doesn't exit the function body, but proceeds to the finally block instead. In this case, the value returned may be different, and done may even be false, if there are more yield expressions within the finally block.

And the practice:

function* citiesGen2() {
    yield "Paris";
    try {
        yield "Lyon";
        yield "Porto";
        return "Stockholm";
    }
    finally {
        yield "Lisbon";
        yield "Berlin";
    }
}

cities = citiesGen2();
console.log(cities.next());
console.log(cities.next());
console.log(cities.return("Over"));
console.log(cities.next());
console.log(cities.next());
console.log(cities.next());


// { value: 'Paris', done: false }
// { value: 'Lyon', done: false }
// { value: 'Lisbon', done: false }
// { value: 'Berlin', done: false }
// { value: 'Over', done: true }
// { value: undefined, done: true }

If this feels odd to you, you're not alone :-D. This is quite different from Python, where a call to close() always finishes the generator, even if we are catching the Exception and returning something from it.

generator.close()

Raises a GeneratorExit at the point where the generator function was paused. If the generator function catches the exception and returns a value, this value is returned from close(). If the generator function is already closed, or raises GeneratorExit (by not catching the exception), close() returns None. If the generator yields a value, a RuntimeError is raised. If the generator raises any other exception, it is propagated to the caller. If the generator has already exited due to an exception or normal exit, close() returns None and has no other effect.

Same as Python, JavaScript generators also have a throw() method, and again, I see no much use for it.