It’s just a more elegant syntax of getting the promise result than promise.then, easier to read and write.

Just like defer, async is an attribute for the classic script tag when we use it to include external script. For example, this will not work: But we can wrap it into an anonymous async function, like this: Like promise.then, await allows us to use thenable objects (those with a callable then method). Just replace .catch with try...catch inside demoGithubUser and add async/await where needed: We have a “regular” function. So in this article we will talk about how to optimize our script tags with both. PHP If you want to report an error, or if you want to make a suggestion, do not hesitate to send us an e-mail: , W3Schools is optimized for learning, testing, and training. Finally I would like to bring all our learned knowledge into a bigger example, where you can clearly see the properties of all possibilities to include our scripts. If a promise resolves normally, then await promise returns the result. Parsing is done before the script is even downloaded, so the page appears to the user way before the previous example. Below you can find the “rethrow” example from the chapter Promises chaining. Usually it’s just the wrong order, because a HTML document is read and executed by the browser from top to bottom — at least that’s the standard for our script-tags, which can contain JavaScript code themselves, or refer to an external file. When we use async/await, we rarely need .then, because await handles the waiting for us.

So the querySelector cannot access the element, because it practically does not exist yet. It’s surprisingly easy to understand and use. If we now look at the section of the network tab, we see almost the same as when we implemented the whole thing with async. Defer is perfect for this, because the execution order is always the same as the order in which we include the scripts from top to bottom in the DOM — regardless of the size of the scripts. Here’s a demo Thenable class; the await below accepts its instances: If await gets a non-promise object with .then, it calls that method providing the built-in functions resolve and reject as arguments (just as it does for a regular Promise executor). Let’s start with the async keyword. Each of the scripts simply contains a console.log to make clear which script it is & a querySelector to access the h1-tag in the DOM as it is in our index.html which is shown below. You can find all of this from our homepage at plainenglish.io — show some love by giving our publications a follow and subscribing to our YouTube channel! I have created a small example with the following file structure. If we include the two scripts as shown above without any async or defer, the library will always be executed or available first. Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness of all content. But if you have (say) two scripts that aren’t interdependent, marking each with the async flag lets the browser run them in either order, and so if (say) the first one is being being slow to download, the browser is free to go ahead and run the second one …

How to call async from it and use its result? Beside async there is also defer, with which we can influence the loading behavior of our scripts. So we just have to include the async as attribute, just like it works with defer, but more about that later. In our case it doesn’t matter. Especially if the script is only an analysis tool like google analytics, and therefore has no added value for the user, we should integrate it with async for performance and security reasons. Basically this attribute is ideal for all scripts that are guaranteed to access the DOM, and must do so successfully at all costs. It’s surprisingly easy to understand and use. There’s a special syntax to work with promises in a more comfortable fashion, called “async/await”. The Process of Learning How to Use Observables in Angular is Terrible. Did you know that we have four publications and a YouTube channel? With defer they are guaranteed to be executed only when the DOM is ready. If you can't understand something in the article – please elaborate.

Here is the output of the browser console in Chrome and Firefox. In today’s world, where JavaScript plays a key role in the dynamics of modern websites, this means that the page is already visible at this point in time, but the user cannot necessarily interact with it, because the scripts required to process events or inputs, for example, have yet to be loaded and executed. Other values are wrapped in a resolved promise automatically. Simple enough, right?