Asynchronous JavaScript
To allow us to understand what asynchronous JavaScript is, we ought to start off by making sure we understand what synchronous JavaScript is. This section recaps some of the information we saw in the previous article.
A lot of the functionality we have looked at in previous learning area modules is synchronous — you run some code, and the result is returned as soon as the browser can do so. Let's look at a simple example
const btn = document.querySelector('button');
btn.addEventListener('click', () => {
alert('You clicked me!');
let pElem = document.createElement('p');
pElem.textContent = 'This is a newly-added paragraph.';
document.body.appendChild(pElem);
});
In this block, the lines are executed one after the other:
- We grab a reference to a <button> element that is already available in the DOM.
- We add a click event listener to it so that when the button is clicked:
- An alert() message appears.
- Once the alert is dismissed, we create a element.
- We then give it some text content.
- Finally, we append the paragraph to the document body.
For reasons illustrated earlier (e.g. related to blocking), many Web API features now use asynchronous code to run, especially those that access or fetch some kind of resource from an external device, such as fetching a file from the network, accessing a database and returning data from it, accessing a video stream from a web cam, or broadcasting the display to a VR headset.
Why is this difficult to get to work using synchronous code? Let's look at a quick example. When you fetch an image from a server, you can't return the result immediately. That means that the following (pseudocode) wouldn't work:
let response = fetch('myImage.png'); // fetch is asynchronous
let blob = response.blob();
// display your image blob in the UI somehow
That's because you don't know how long the image will take to download, so when you come to run the second line it will throw an error (possibly intermittently, possibly every time) because the response is not yet available. Instead, you need your code to wait until the response is returned before it tries to do anything else to it.
There are two main types of asynchronous code style you'll come across in JavaScript code, old-style callbacks and newer promise-style code. In the below sections we'll review each of these in turn.
Async callbacks are functions that are specified as arguments when calling a function which will start executing code in the background. When the background code finishes running, it calls the callback function to let you know the work is done, or to let you know that something of interest has happened. Using callbacks is slightly old-fashioned now, but you'll still see them in use in a number of older-but-still-commonly-used APIs.
An example of an async callback is the second parameter of the addEventListener() method (as we saw in action above):
btn.addEventListener('click', () => {
alert('You clicked me!');
let pElem = document.createElement('p');
pElem.textContent = 'This is a newly-added paragraph.';
document.body.appendChild(pElem);
});
The first parameter is the type of event to be listened for, and the second parameter is a callback function that is invoked when the event is fired. When we pass a callback function as an argument to another function, we are only passing the function's reference as an argument, i.e, the callback function is not executed immediately. It is “called back” (hence the name) asynchronously somewhere inside the containing function’s body. The containing function is responsible for executing the callback function when the time comes. You can write your own function containing a callback easily enough. Let's look at another example that loads a resource via the XMLHttpRequest API (run it live, and see the source):
function loadAsset(url, type, callback) {
let xhr = new XMLHttpRequest();
xhr.open('GET', url);
xhr.responseType = type;
xhr.onload = function() {
callback(xhr.response);
};
xhr.send();
}
function displayImage(blob) {
let objectURL = URL.createObjectURL(blob);
let image = document.createElement('img');
image.src = objectURL;
document.body.appendChild(image);
}
loadAsset('coffee.jpg', 'blob', displayImage);
Promises are the new style of async code that you'll see used in modern Web APIs. A good example is the fetch() API, which is basically like a modern, more efficient version of XMLHttpRequest. Let's look at a quick example, from our Fetching data from the server article:
fetch('products.json').then(function(response) {
return response.json();
}).then(function(json) {
products = json;
initialize();
}).catch(function(err) {
console.log('Fetch problem: ' + err.message);
});
Here we see fetch() taking a single parameter — the URL of a resource you want to fetch from the network — and returning a promise. The promise is an object representing the completion or failure of the async operation. It represents an intermediate state, as it were. In essence, it's the browser's way of saying "I promise to get back to you with the answer as soon as I can," hence the name "promise." This concept can take practice to get used to; it feels a little like Schrödinger's cat in action. Neither of the possible outcomes have happened yet, so the fetch operation is currently waiting on the result of the browser trying to complete the operation at some point in the future. We've then got three further code blocks chained onto the end of the fetch():
- Two then() blocks. Both contain a callback function that will run if the previous operation is successful, and each callback receives as input the result of the previous successful operation, so you can go forward and do something else to it. Each .then() block returns another promise, meaning that you can chain multiple .then() blocks onto each other, so multiple asynchronous operations can be made to run in order, one after another.
- The catch() block at the end runs if any of the .then() blocks fail — in a similar way to synchronous try...catch blocks, an error object is made available inside the catch(), which can be used to report the kind of error that has occurred. Note however that synchronous try...catch won't work with promises, although it will work with async/await, as you'll learn later on.
In its most basic form, JavaScript is a synchronous, blocking, single-threaded language, in which only one operation can be in progress at a time. But web browsers define functions and APIs that allow us to register functions that should not be executed synchronously, and should instead be invoked asynchronously when some kind of event occurs (the passage of time, the user's interaction with the mouse, or the arrival of data over the network, for example). This means that you can let your code do several things at the same time without stopping or blocking your main thread.
Whether we want to run code synchronously or asynchronously will depend on what we're trying to do.
There are times when we want things to load and happen right away. For example when applying some user-defined styles to a webpage you'll want the styles to be applied as soon as possible.
If we're running an operation that takes time however, like querying a database and using the results to populate templates, it is better to push this off the main thread and complete the task asynchronously. Over time, you'll learn when it makes more sense to choose an asynchronous technique over a synchronous one.