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Difference between async and defer attributes in script tags

Lately you might have come across script tags in your html documents with the async and/or defer attributes. In this article, we will explain these terms so that you can decide when to use it in your application.

NOTE: Consider checking browser support before using these features in your application.

Lets start with a regular script tag having no attributes.

<script src="myfile.js"></script>

When the browser comes across the above line in your markup, this is what happens.

Pause parsing the document.
Make a request to fetch the file.
Execute the script after it has been downloaded.
Resume parsing the document.

As you can tell, this leads to a very bad user experience since the browser isint really doing anything useful while the script file is downloading. To avoid this, most applications place the script tag at the bottom of the html page, which is a much better way to solve this problem. In fact, this is exactly what the async attribute is for.

Async

When you add the async attribute to your script tag, the following will happen.

<script src="myfile1.js" async></script>
<script src="myfile2.js" async></script>

Make parallel requests to fetch the files.
Continue parsing the document as if it was never interrupted.
Execute the individual scripts the moment the files are downloaded.

The great thing of this flow is that scripts can download in parallel while the document is being parsed. But there's a caveat to this and thats the third point - the script will be executed the moment it is downloaded. This can be a non-issue if a script is completely self contained. However, in many situations, scripts may depend on other scripts to have done some initialization before they can execute. e.g. jquery plugins require the jquery variable to already exist on the page.

NOTE: Scripts that you programmatically insert into the DOM are async by default, unless you explicitly set their async attribute to false at insertion time.

Defer

Defer is very similar to async with one major differerence. Here's what happens when a browser encounters a script with the defer attribute.

<script src="myfile1.js" defer></script>
<script src="myfile2.js" defer></script>

Make parallel requests to fetch the individual files.
Continue parsing the document as if it was never interrupted.
Finish parsing the document even if the script files have downloaded.
Execute each script in the order they were encountered in the document.

As you can tell, defer is pretty much what you want to do in your files. However, due to limited browser support, its not a viable option at the time of writing.

NOTE: The async and defer attributes are ignored for scripts having no src attribute.

Until we live in a better world, the good ol' technique of placing scripts at the bottom of your document should work just fine for scripts that are dependednt on each other. As discussed earlier, the best use case for using the async attribute in scripts is for external scripts that are completely self contained.