Understanding Generators in JavaScript

Understanding Generators in JavaScript

Arrow functions, classes and modules quickly became popular and are heavily used in modern JavaScript programming. Do you know what generators are? Generators are all about iteration. So, let’s see how iteration evolved in JavaScript. Generators are produced by a special type of function, called generator function. Even when you explicitly call a generator function, its instructions are not executed. A generator is both an iterable and an iterator. When the next() method is called, the execution proceeds until a yield instruction is found.

In 2015, a rich set of new features was introduced to the JavaScript community. Things like arrow functions, classes and modules quickly became popular and are heavily used in modern JavaScript programming. Some other features are less known and used by developers nowadays. Do you know what generators are? This is exactly what we will learn in this article.

A brief introduction

Before understanding generators, we need to see a few other things first. Generators are all about iteration. So, let’s see how iteration evolved in JavaScript. This way, we can learn what benefits could generators bring when properly used.

Iteration

In JavaScript, arrays are the most common thing people iterate over. So, let’s take them as an example. There are multiple ways of iterating over an array. Here are the four most common:

1) Using the forEach instance method

someArray.forEach(item => console.log(item));

2) Using while loops

let i = 0;
while (i < someArray.length) {
    const item = someArray[i];
    console.log(item);
    i++;
}

3) Using for loops

for (let i = 0; i < someArray.length; i++) {
    const item = someArray[i];
    console.log(item);
}

4) Using for-of loops

for (let item of someArray) {
    console.log(item);
}

It looks clear that the forEach method and the for-of loop are the friendliest options. The other two are more verbose and expect us to deal with indexes manually. The forEach method is defined in the Array’s prototype. The for-of loop allows us to iterate over arrays, maps, sets and even strings. Yes, strings as well:

for (let letter of 'abcdef') {
    console.log(letter); 
}
//=> 'a'
//=> 'b'
//=> 'c'
//=> 'd'
//=> 'e'
//=> 'f'

Iterators

Any object that implements the iterator protocol is an iterator. The iterator protocol is really simple to understand and adhere to. It expects the object to have a next() method. When called, it should move to the next item in the sequence and return an object. This object must have at least two properties: done and value. The done property is a boolean that indicates whether the iterator is past the end. The value property, as the name suggests, is the value at the current position.

Let’s see a simple example. This function creates iterators with a sequence of squared numbers:

function buildSquaredNumbersIterator(from = 1, to = 10) {
    let count = from - 1;

    return {
        next: () => {
            if (count >= to) return { done: true };

            count++;
            return { value: count * count, done: false };
        }
    };
}

To create an iterator using the above function, you just need to call it:

const iterator = buildSquaredNumbersIterator(2, 9);
console.log(iterator.next()); //=> { value: 4, done: true }
console.log(iterator.next()); //=> { value: 9, done: true }
console.log(iterator.next()); //=> { value: 16, done: true }
console.log(iterator.next()); //=> { value: 25, done: true }
console.log(iterator.next()); //=> { value: 36, done: true }
console.log(iterator.next()); //=> { value: 49, done: true }
console.log(iterator.next()); //=> { value: 64, done: true }
console.log(iterator.next()); //=> { value: 81, done: true }
console.log(iterator.next()); //=> { done: false }

Naturally, you could simply use a loop to iterate over it:

const iterator = buildSquaredNumbersIterator(2, 9);
let res = iterator.next();

while (!res.done) {
    console.log(result.value); 
    res = it.next();
}

Ok, but… could we use our custom iterator with a for-of loop? Well, the answer is no. To be compatible with for-of loops, an object must adhere to the iterable protocol.

Iterables

To be considered an iterable, an object must implement the iterable protocol. It expects the object to have a method whose key is the value of _Symbol.iterator. _This method will be called with no arguments and should return an iterator.

Let’s adapt our custom iterator to adhere to this protocol:

function buildSquaredNumbersIterator(from = 1, to = 10) {
    let count = from - 1;

    return {
        next: () => {
            if (count >= to) return { done: true };

            count++;
            return { value: count * count, done: false };
        },
        [Symbol.iterator]: function() { 
            return this; 
        }
    };
}

The only difference here is the method with the Symbol.iterator key. But why is it just returning this? This is simple to understand. As we saw, the iterable protocol expects this special method to return an iterator. Note that we’re adding this method to an object that adheres to the iterator protocol. This way, we can simply return this, that will point to the object itself. By doing so, our iterator becomes an iterable. This will make it usable in for-of loops:

const numbers = buildSquaredNumbersIterator(2, 9);

for (let n of numbers) {
    console.log(n);
}
//=> 4
//=> 9
//=> 16
//=> 25
//=> 36
//=> 49
//=> 64
//=> 81

Generators

Our custom iterable/iterator works really well. However, we could have implemented the exact same functionality in a very simpler way by using ES2015’s generator functions. But before refactoring our iterator, let’s understand what a generator is.

  • Generators are produced by a special type of function, called generator function.
  • A generator is both an iterable and an iterator.
  • Even when you explicitly call a generator function, its instructions are not executed.
  • When the next() method is called, the execution proceeds until a yield instruction is found.
  • The yield instruction returns the current value in the iteration sequence.
  • Once a yield instruction is found and a value is returned, the execution is paused. After that, it waits for a new call to the next() method.
  • This cycle repeats until no more yield instructions are found.
  • The iterator comes to an end.

Here is a simple generator function that yields three values:

function *buildHardcodedValuesGenerator() {
    console.log('One');
    yield 1;
    console.log('Two');
    yield 2;
    console.log('Three');
    yield 3;
    console.log('The end');
}

const generator = buildHardcodedValuesGenerator();
console.log('Returned value:', generator.next().value);
//=> 'One'
//=> Returned value: 1
console.log('Returned value:', generator.next().value);
//=> 'Two'
//=> Returned value: 2
console.log('Returned value:', generator.next().value);
//=> 'Three'
//=> Returned value: 3
console.log('Returned value:', generator.next().value);
//=> 'The end'
//=> Returned value: undefined

As you can see, the body of our generator function is not executed when it is first called. It is also clear that every time we call the next() method, the execution proceeds until the next yield is found. When this happens, it pauses the execution and yields a value. When no more yields are found, the next() method returns an object with a value of undefined and a done flag set to true.

Refactoring our generator

Now, let’s see how our iterator could be converted to a generator:

function *buildSquaredNumbersIterator(from = 1, to = 10) {
    for (let count = from; count <= to; count++) {
        yield count * count;
    }
}

This looks a lot simpler than our previous version. We don’t have to handle the internal state. Nor do we need to adhere to some protocol. We just call yield for every value that should be returned. Since generators are iterables, we could iterate over it with a for-of loop as well:

const numbersGenerator = buildSquaredNumbersIterator(4, 11);

for (let number of numbersGenerator) {
    console.log(number);
}
//=> 16
//=> 25
//=> 36
//=> 49
//=> 64
//=> 81
//=> 100
//=> 121

Conclusion

  • An iterator is any object that adheres to the iterator protocol.
  • The iterator protocol expects an object to have a next() method. This method should return an object with the value at the current position and a done flag.
  • An iterable is any object that adheres to the iterable protocol.
  • The iterable protocol expects an object to have a method whose key is Symbol.iterator. This method should return a valid iterator.
  • A generator function is a special type of function that builds generators and contains their logic.
  • A generator is an object that adheres to both iterator and iterable protocols.

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JavaScript developers should you be using Web Workers?

JavaScript developers should you be using Web Workers?

Do you think JavaScript developers should be making more use of Web Workers to shift execution off of the main thread?

Originally published by David Gilbertson at https://medium.com

So, Web Workers. Those wonderful little critters that allow us to execute JavaScript off the main thread.

Also known as “no, you’re thinking of Service Workers”.

Photo by Caleb Jones on Unsplash

Before I get into the meat of the article, please sit for a lesson in how computers work:

Understood? Good.

For the red/green colourblind, let me explain. While a CPU is doing one thing, it can’t be doing another thing, which means you can’t sort a big array while a user scrolls the screen.

This is bad, if you have a big array and users with fingers.

Enter, Web Workers. These split open the atomic concept of a ‘CPU’ and allow us to think in terms of threads. We can use one thread to handle user-facing work like touch events and rendering the UI, and different threads to carry out all other work.

Check that out, the main thread is green the whole way through, ready to receive and respond to the gentle caress of a user.

You’re excited (I can tell), if we only have UI code on the main thread and all other code can go in a worker, things are going to be amazing (said the way Oprah would say it).

But cool your jets for just a moment, because websites are mostly about the UI — it’s why we have screens. And a lot of a user’s interactions with your site will be tapping on the screen, waiting for a response, reading, tapping, looking, reading, and so on.

So we can’t just say “here’s some JS that takes 20ms to run, chuck it on a thread”, we must think about where that execution time exists in the user’s world of tap, read, look, read, tap…

I like to boil this down to one specific question:

Is the user waiting anyway?

Imagine we have created some sort of git-repository-hosting website that shows all sorts of things about a repository. We have a cool feature called ‘issues’. A user can even click an ‘issues’ tab in our website to see a list of all issues relating to the repository. Groundbreaking!

When our users click this issues tab, the site is going to fetch the issue data, process it in some way — perhaps sort, or format dates, or work out which icon to show — then render the UI.

Inside the user’s computer, that’ll look exactly like this.

Look at that processing stage, locking up the main thread even though it has nothing to do with the UI! That’s terrible, in theory.

But think about what the human is actually doing at this point. They’re waiting for the common trio of network/process/render; just sittin’ around with less to do than the Bolivian Navy.

Because we care about our users, we show a loading indicator to let them know we’ve received their request and are working on it — putting the human in a ‘waiting’ state. Let’s add that to the diagram.

Now that we have a human in the picture, we can mix in a Web Worker and think about the impact it will have on their life:

Hmmm.

First thing to note is that we’re not doing anything in parallel. We need the data from the network before we process it, and we need to process the data before we can render the UI. The elapsed time doesn’t change.

(BTW, the time involved in moving data to a Web Worker and back is negligible: 1ms per 100 KB is a decent rule of thumb.)

So we can move work off the main thread and have a page that is responsive during that time, but to what end? If our user is sitting there looking at a spinner for 600ms, have we enriched their experience by having a responsive screen for the middle third?

No.

I’ve fudged these diagrams a little bit to make them the gorgeous specimens of graphic design that they are, but they’re not really to scale.

When responding to a user request, you’ll find that the network and DOM-manipulating part of any given task take much, much longer than the pure-JS data processing part.

I saw an article recently making the case that updating a Redux store was a good candidate for Web Workers because it’s not UI work (and non-UI work doesn’t belong on the main thread).

Chucking the data processing over to a worker thread sounds sensible, but the idea struck me as a little, umm, academic.

First, let’s split instances of ‘updating a store’ into two categories:

  1. Updating a store in response to a user interaction, then updating the UI in response to the data change
  2. Not that first one

If the first scenario, a user taps a button on the screen — perhaps to change the sort order of a list. The store updates, and this results in a re-rendering of the DOM (since that’s the point of a store).

Let me just delete one thing from the previous diagram:

In my experience, it is rare that the store-updating step goes beyond a few dozen milliseconds, and is generally followed by ten times that in DOM updating, layout, and paint. If I’ve got a site that’s taking longer than this, I’d be asking questions about why I have so much data in the browser and so much DOM, rather than on which thread I should do my processing.

So the question we’re faced with is the same one from above: the user tapped something on the screen, we’re going to work on that request for hopefully less than a second, why would we want to make the screen responsive during that time?

OK what about the second scenario, where a store update isn’t in response to a user interaction? Performing an auto-save, for example — there’s nothing more annoying than an app becoming unresponsive doing something you didn’t ask it to do.

Actually there’s heaps of things more annoying than that. Teens, for example.

Anyhoo, if you’re doing an auto-save and taking 100ms to process data client-side before sending it off to a server, then you should absolutely use a Web Worker.

In fact, any ‘background’ task that the user hasn’t asked for, or isn’t waiting for, is a good candidate for moving to a Web Worker.

The matter of value

Complexity is expensive, and implementing Web Workers ain’t cheap.

If you’re using a bundler — and you are — you’ll have a lot of reading to do, and probably npm packages to install. If you’ve got a create-react-app app, prepare to eject (and put aside two days twice a year to update 30 different packages when the next version of Babel/Redux/React/ESLint comes out).

Also, if you want to share anything fancier than plain data between a worker and the main thread you’ve got some more reading to do (comlink is your friend).

What I’m getting at is this: if the benefit is real, but minimal, then you’ve gotta ask if there’s something else you could spend a day or two on with a greater benefit to your users.

This thinking is true of everything, of course, but I’ve found that Web Workers have a particularly poor benefit-to-effort ratio.

Hey David, why you hate Web Workers so bad?

Good question.

This is a doweling jig:

I own a doweling jig. I love my doweling jig. If I need to drill a hole into the end of a piece of wood and ensure that it’s perfectly perpendicular to the surface, I use my doweling jig.

But I don’t use it to eat breakfast. For that I use a spoon.

Four years ago I was working on some fancy animations. They looked slick on a fast device, but janky on a slow one. So I wrote fireball-js, which executes a rudimentary performance benchmark on the user’s device and returns a score, allowing me to run my animations only on devices that would render them smoothly.

Where’s the best spot to run some CPU intensive code that the user didn’t request? On a different thread, of course. A Web Worker was the correct tool for the job.

Fast forward to 2019 and you’ll find me writing a routing algorithm for a mapping application. This requires parsing a big fat GeoJSON map into a collection of nodes and edges, to be used when a user asks for directions. The processing isn’t in response to a user request and the user isn’t waiting on it. And so, a Web Worker is the correct tool for the job.

It was only when doing this that it dawned on me: in the intervening quartet of years, I have seen exactly zero other instances where Web Workers would have improved the user experience.

Contrast this with a recent resurgence in Web Worker wonderment, and combine that contrast with the fact that I couldn’t think of anything else to write about, then concatenate that combined contrast with my contrarian character and you’ve got yourself a blog post telling you that maybe Web Workers are a teeny-tiny bit overhyped.

Thanks for reading

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Further reading

An Introduction to Web Workers

JavaScript Web Workers: A Beginner’s Guide

Using Web Workers to Real-time Processing

How to use Web Workers in Angular app

Using Web Workers with Angular CLI