JS generator functions in 30 seconds

The generator concept exists in most languages, and is a very useful abstraction. Definitely learn and use it. Here’s what it looks like in Javascript:

function *aGeneratorFunction() {
    console.log('stage 1');
    yield 123;
    console.log('stage 2');
    yield "almost there";
    console.log('stage 3, done');
    return {foo: "bar"};
}

Make a “Generator” object and run it:

let gen = aGeneratorFunction();
gen.next(); // {value: 123, done: false}
> stage 1
gen.next(); // {value: "almost there", done: false}
> stage 2
gen.next(); // {value: {foo: "bar"}, done: true}
> stage 3, done

Generally speaking, you should yield the same type every time. I only used different types to get the idea across.

Example of how generators can be useful: they are evaluated lazily - i.e. only as the consumer asks for the next value (.next()). This implies O(1) memory usage for generating series of any length (even infinite). More depth on the Wikipedia page .