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Cover image for Generators in JavaScript, Part II - Simple use-case

Generators in JavaScript, Part II - Simple use-case

mpodlasin profile image Mateusz Podlasin Originally published at mpodlasin.com ・6 min read

The behavior of generators that we've described in the previous article is not complicated, but it is certainly surprising and might be difficult to grasp at the very beginning.

So in this article, instead of introducing more concepts, we will pause a bit and use only what we've learned to this point while discovering a cool use-case for generators.

Let's say that we have a function like this:

function maybeAddNumbers() {
    const a = maybeGetNumberA();
    const b = maybeGetNumberB();

    return a + b;
}
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Functions maybeGetNumberA and maybeGetNumberB return numbers, but sometimes they might also return null or undefined. That's what "maybe" in their names signalizes.

When that's the case, we shouldn't try to add those values (for example a number and null), but rather bail out immediately and just return, let's say, null again. After all, it's better to return null here, rather than some unpredictable value resulting from adding null/undefined with a number or with another null/undefined.

So we have to add a check that makes sure those numbers are actually defined:

function maybeAddNumbers() {
    const a = maybeGetNumberA();
    const b = maybeGetNumberB();

    if (a === null || a === undefined || b === null || b === undefined) {
        return null;
    }

    return a + b;
}
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This works okay, but if a is either a null or an undefined, there is really no point in calling the maybeGetNumberB function at all. That's because we already know that we will return a null anyways.

So let's rewrite the function again:

function maybeAddNumbers() {
    const a = maybeGetNumberA();

    if (a === null || a === undefined) {
        return null;
    }

    const b = maybeGetNumberB();

    if (b === null || b === undefined) {
        return null;
    }

    return a + b;
}
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Uuuh. From an easy to read 3-liner, this quickly grew to 10 lines of code (not counting the empty lines). This function is now filled with if cases, which you have to get through in order to understand what it does.

And this is just a toy example! You can imagine that in actual codebases, which contain much more complex logic, those checks would become even more complicated.

So what if we could use generators here and bring back the code to its simpler form?

Take a look at this:

function* maybeAddNumbers() {
    const a = yield maybeGetNumberA();
    const b = yield maybeGetNumberB();

    return a + b;
}
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What if we could give that yield <something> expression the functionality of checking if <something> is an actual value and not null or undefined?

If it turned out that <something> is null or undefined, we would just bail early and return null, exactly like in the more verbose version of our code.

This way we could write code that looks almost as if it deals only with actual, defined values.
It's the generator itself that would check for you if that's really the case and it would act accordingly! Sounds magical, doesn't it?

And yet it's not only possible but also very easy to write!

Of course, generators themselves don't possess this functionality. They just return iterators and optionally allow you to inject some values back into the generator.

So we will have to write a wrapper - let's call it runMaybe - which will give the generator this capability.

So instead of calling the function directly:

const result = maybeAddNumbers();
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We will be calling it as an argument to that wrapper:

const result = runMaybe(maybeAddNumbers());
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This is a pattern that you will see incredibly often with generators.

Generators by themselves don't do much, but by writing custom wrappers like this one, you can grant generators custom behaviors! And that's precisely what we will do right now.

So runMaybe obviously is a function and it accepts one argument - an iterator produced by the generator:

function runMaybe(iterator) {

}
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We will run this iterator in a while loop. In order to do that, we need to call the iterator for the first time and start checking its done property:

function runMaybe(iterator) {
    let result = iterator.next();

    while(!result.done) {

    }
}
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Now inside a loop we have two options. If result.value is null or undefined we want to break the iteration process immediately and return null. Let's do that:

function runMaybe(iterator) {
    let result = iterator.next();

    while(!result.done) {
        if (result.value === null || result.value === undefined) {
            return null;
        }
    }
}
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You can see that we are immediately stopping the iteration with the return and we are returning a null from our wrapper.

If however result.value is an actual, defined value, we want to "give it back" to the generator.

For example in yield maybeGetNumberA(), if it turns out that maybeGetNumberA() is actually a number, we just want to replace yield maybeGetNumberA() with the value of the number itself.

Even more specifically, if maybeGetNumberA() evaluated to, say, number 5, we would like to change const a = yield maybeGetNumberA(); into const a = 5;. As you can see, we don't want to change the yielded value in any way, but simply pass it back to the generator.

We remember that we can replace yield <something> with some value by passing that value as an argument to the iterators next method. So let's do that!

function runMaybe(iterator) {
    let result = iterator.next();

    while(!result.done) {
        if (result.value === null || result.value === undefined) {
            return null;
        }

        // we are passing result.value back
        // to the generator
        result = iterator.next(result.value)
    }
}
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And as you can see, the new result gets now stored in the result variable again. We've specifically declared result with let so that it's possible.

We are almost there - if at any point our generator encounters a null/undefined when yielding a value, we just return a null from our runMaybe wrapper.

But we need to return something also if the iteration process finishes without encountering any null/undefined values. After all, if we receive two actual numbers in our generator, we want to return their sum from the wrapper!

Our maybeAddNumbers generator ends with a return statement.

We remember that return <something> in a generator causes its iterator to return an object { value: <something>, done: true } from a next call.

When this happens, while loop will stop running, because done property will be set to true. But that last returned value (in our specific case a + b value) still will be stored in the result.value property! So at the end we can simply return it:

function runMaybe(iterator) {
    let result = iterator.next();

    while(!result.done) {
        if (result.value === null || result.value === undefined) {
            return null;
        }

        result = iterator.next(result.value)
    }

    // just return the last value
    // after the iterator is done
    return result.value;
}
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And... that's it!

Let's create dummy maybeGetNumberA and maybeGetNumberB functions. Let's make them return actual numbers first:

const maybeGetNumberA = () => 5;
const maybeGetNumberB = () => 10;
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If we run our code now and log the results:

function* maybeAddNumbers() {
    const a = yield maybeGetNumberA();
    const b = yield maybeGetNumberB();

    return a + b;
}

const result = runMaybe(maybeAddNumbers());

console.log(result);
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We will see - as expected - number 15 in the console.

Let's however change one of the added numbers to null:

const maybeGetNumberA = () => null;
const maybeGetNumberB = () => 10;
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Now running the code logs null!

It was however important to us to make sure that maybeGetNumberB function doesn't called when the first function - maybeGetNumberA - returns null/undefined. So let's double-check if we really succeeded.

We can do it simply by adding a console.log to the second function:

const maybeGetNumberA = () => null;
const maybeGetNumberB = () => {
    console.log('B');
    return 10;
}
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If we wrote our runMaybe helper correctly, the letter B should not appear in the console when running this example.

And indeed, if you run the code now, you will simply see null in the console, and nothing else. This means that our helper actually stops running the generator after it encounters a null/undefined value.

Our code also works as intended - by logging null - in any of those combinations:

const maybeGetNumberA = () => undefined;
const maybeGetNumberB = () => 10;
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const maybeGetNumberA = () => 5;
const maybeGetNumberB = () => null;
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const maybeGetNumberA = () => undefined;
const maybeGetNumberB = () => null;
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etc.

The power of this example doesn't lay however in running this particular code.

It lays in the fact that we've created a general helper, which can handle any generator that potentially yields null/undefined values.

For example if we wrote a more complex function:

function* maybeAddFiveNumbers() {
    const a = yield maybeGetNumberA();
    const b = yield maybeGetNumberB();
    const c = yield maybeGetNumberC();
    const d = yield maybeGetNumberD();
    const e = yield maybeGetNumberE();

    return a + b + c + d + e;
}
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We can run it in our runMaybe wrapper as well without any issue!

In fact, our wrapper doesn't even rely on the fact that in our examples those functions are returning numbers. Note that in runMaybe we don't mention the number type at all. So no matter what kind of values you are using in your generator - numbers, strings, objects, arrays, more complex data structures - it will still work with our helper!

This is exactly what developers find exciting about generators. They allow you to introduce custom functionality to the code that looks very regular (apart from those yield calls of course). You just need to create a wrapper that iterates over a generator in a particular way. This way, the wrapper basically "grants" the generator custom functionality!

And that functionality could be literally anything you want. Generators introduce potentially endless possibilities and the only limitation are our imaginations!

And in the following articles, we will keep exploring those possibilities, especially in combination with React. So if this sounds interesting to you, follow me on Twitter to not miss those future articles.

Thanks for reading!

Discussion (1)

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whoadarshpandya profile image
Adarsh Pandya

Awesome

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