Asynchronous processing is one of the most important aspects of the JavaScript language. JavaScript is a single-threaded language, so it needs a mechanism to deal with the actions like performing time-intensive and potentially unreliable activities like making REST API calls, reading files from the disk, or interacting with user input.

In JavaScript the mechanism used is an event loop, which processes a callback queue in order from oldest item to newest. Each item in the queue is a message that’s processed completely before moving on to the next message. Each message is a function that returns a response to code elsewhere in the application. Functions used in this way are called callback functions.

Because of callbacks, JavaScript can use a number of techniques for getting around its single-threaded nature to implement asynchronous processing. In this and the next article we will try to explain these concepts, So buckle up it's gonna be a bumpy ride 😁.

In this one we are going to see JavaScript callbacks. We are going to explore what they are, why and where we use them. We will also see the potential issues with Callbacks and in the next article we are going to explore what promises are, the issues associated with using them and how async await solves those issues.

Callbacks

I'm going to assume you know exactly 0 about callbacks. If I'm assuming wrong, just scroll down a bit.

When I was first learning JavaScript, it helped me to think about functions as machines. These machines can do anything you want them to. They can even accept input and return a value. Each machine has a button on it that you can press when you want the machine to run.

function add(x, y) {
    return x + y;
}

add(2, 3); // Press the button, run the machine.

Whether I press the button, you press the button, or someone else presses the button it doesn't matter. Whenever the button is pressed, like it or not, the machine is going to run.

function add(x, y) {
    return x + y;
}

const me = add;
const you = add;
const someoneElse = add;

me(2, 3); // Press the button, run the machine.
you(2, 3); // Press the button, run the machine.
someoneElse(2, 3); // Press the button, run the machine.

In the code above we assign the add function to three different variables, me, you, and someoneElse. It's important to note that the original add and each of the variables we created are pointing to the same spot in memory. They're literally the exact same thing under different names. So when we invoke me, you, or someoneElse, it's as if we're invoking add. Now, what if we take our add machine and pass it to another machine? Remember, it doesn't matter who presses the () button, if it's pressed, it's going to run.

function add(x, y) {
    return x + y;
}

function addFive(x, addReference) {
    return addReference(x, 5); // 15 - Press the button, run the machine.
}

addFive(10, add); // 15

Your brain might have got a little weird on this one, nothing new is going on here though. Instead of "pressing the button" on add, we pass add as an argument to addFive, rename it addReference, and then we "press the button" or invoke it.

This highlights some important concepts of the JavaScript language. First, just as you can pass a string or a number as an argument to a function, so too can you pass a reference to a function as an argument. When you do this the function you're passing as an argument is called a callback function and the function you're passing the callback function to is called a higher order function.

Because vocabulary is important, here's the same code with the variables re-named to match the concepts they're demonstrating.

function add(x, y) {
    return x + y;
}

function higherOrderFunction(x, callback) {
    return callback(x, 5);
}

higherOrderFunction(10, add);

This pattern should look familiar, it's everywhere. If you've ever used any of the JavaScript Array methods, you've used a callback. If you've ever used lodash, you've used a callback. If you've ever used jQuery, you've used a callback.

[1, 2, 3].map((i) => i + 5);

_.filter([1, 2, 3, 4], (n) => n % 2 === 0);

$("#btn").on("click", () => console.log("Callbacks are everywhere"));

In general, there are two popular use cases for callbacks. The first, and what we see in the .map and _.filter
examples, is a nice abstraction over transforming one value into another. We say "Hey, here's an array and a function. Go ahead and get me a new value based on the function I gave you". The second, and what we see in the jQuery example, is delaying execution of a function until a particular time. "Hey, here's this function. Go ahead and invoke it whenever the element with an id of btn is clicked." It's this second use case that we're going to focus on, "delaying execution of a function until a particular time".

Right now we've only looked at examples that are synchronous. As we talked about at the beginning of this post, most of the apps we build don't have all the data they need up front. Instead, they need to fetch external data as the user interacts with the app. We've just seen how callbacks can be a great use case
for this because, again, they allow you to "delay execution of a function until a particular time". It doesn't take much imagination to see how we can adapt that sentence to work with data fetching. Instead of delaying execution of a function until a particular time, we can delay execution of a function until we have the data we need. Here's probably
the most popular example of this, jQuery's getJSON method.

// updateUI and showError are irrelevant.
// Pretend they do what they sound like.
const id = "endalk200";

$.getJSON({
    url: `https://api.github.com/users/${id}`,
    success: updateUI,
    error: showError
});

We can't update the UI of our app until we have the user's data. So what do we do? We say, "Hey, here's an object. If
the request succeeds, go ahead and call success passing it the user's data. If it doesn't, go ahead and call error
passing it the error object. You don't need to worry about what each method does, just be sure to call them when you're
supposed to". This is a perfect demonstration of using a callback for async requests.

At this point, we've learned about what callbacks are and how they can be beneficial both in synchronous and asynchronous code. What we haven't talked yet is the dark side of callbacks. Take a look at this code below. Can you tell what's happening?

// updateUI, showError, and getLocationURL are irrelevant.
// Pretend they do what they sound like.
const id = "endalk200";

$("#btn").on("click", () => {
    $.getJSON({
        url: `https://api.github.com/users/${id}`,
        success: (user) => {
            $.getJSON({
                url: getLocationURL(user.location.split(",")),
                success(weather) {
                    updateUI({ user, weather: weather.query.results });
                },
                error: showError
            });
        },
        error: showError
    });
});

Notice we've added a few more layers of callbacks. First, we're saying don't run the initial AJAX request until the element with an id of btn is clicked. Once the button is clicked, we make the first request. If that request succeeds, we make a second request. If that request succeeds, we invoke the updateUI method passing it the data we got from both requests. Regardless of if you understood the code at first glance or not, objectively it's much harder to read than the code before. This brings us to the topic of "Callback Hell".

As humans, we naturally think sequentially. When you have nested callbacks inside of nested callbacks, it forces you out of your natural way of thinking. Bugs happen when there's a disconnect between how your software is read and how you naturally think.

Like most solutions to software problems, a commonly prescribed approach for making "Callback Hell" easier to consume is to modularize your code.

const getUser = (id, onSuccess, onFailure) => {
    $.getJSON({
        url: `https://api.github.com/users/${id}`,
        success: onSuccess,
        error: onFailure
    });
};

const getWeather = (user, onSuccess, onFailure) => {
    $.getJSON({
        url: getLocationURL(user.location.split(",")),
        success: onSuccess,
        error: onFailure
    });
};

$("#btn").on("click", () => {
    getUser(
        "endalk200",
        (user) => {
            getWeather(
                user,
                (weather) => {
                    updateUI({ user, weather: weather.query.results });
                },
                showError
            );
        },
        showError
    );
});

OK, the function names help us understand what's going on, but is it objectively "better"? Not by much. We've put a band-aid over the readability issue of Callback Hell. The problem still exists that we naturally think sequentially and, even with the extra functions, nested callbacks break us out of that sequential way of thinking.

The next issue of callbacks has to do with inversion of control. When you write a callback, you're assuming that the program you're giving the callback to is responsible and will call it when (and only when) it's supposed to. You're essentially inverting the control of your program over to another
program. When you're dealing with libraries like jQuery, lodash, or even vanilla JavaScript, it's safe to assume that the callback function will be invoked at the correct time with the correct arguments. However, for many third-party libraries, callback functions are the interface for how you interact with them. It's entirely plausible that a third party library could,
whether on purpose or accidentally, break how they interact with your callback.

const criticalFunction = () => {
    // It's critical that this function
    // gets called and with the correct arguments.
};

thirdPartyLib(criticalFunction);

Since you're not the one calling criticalFunction, you have 0 control over when and with what argument it's invoked. Most of the time this isn't an issue, but when it is, it's a big one.

In the next article we are going to explore JavaScript promises and how they can provide a potential solution to the
inversion of control problem.

You can find the next article on JavaScript promises and, async and await here