Can you use both the async and defer attributes on a HTML script tag?

Question

I would like to load in the following JavaScript code using both defer and async:

<script defer async src="/js/somescript.js"></script>

Since defer is supported by Internet Explorer 5.5+, as you can see at CanIUse.com, I would like to gracefully fallback to using defer if async is not available. Async I think is better to use when it is available but its not supported until Internet Explorer 10.

My question is therefore is the above code valid HTML? If not, is it possible to create this situation using JavaScript of gracefully falling back to using defer on a script when async is not available?

Answer 1

From the specification: https://www.w3.org/TR/2011/WD-html5-20110525/scripting-1.html#attr-script-async

The defer attribute may be specified even if the async attribute is specified, to cause legacy Web browsers that only support defer (and not async) to fall back to the defer behavior instead of the synchronous blocking behavior that is the default.

(Check the reference link below to see a visual representation of the differences between normal scripts and scripts with defer and async)

---

References:

• async vs defer attributes
• Efficiently load JavaScript with defer and async

Answer 2

The question is, what would you expect it to do? If both async and defer are present I would expect the script to be deferred and only executed after DOMContentLoaded when the browser is idle, but if I'm reading the specification right it looks like defer is ignored if async is set and the script is loaded asynchronously, so will execute as soon as it's available, which may well be before DOMContentLoaded and may block other resources.

There are three possible modes that can be selected using these attributes. If the async attribute is present, then the script will be executed asynchronously, as soon as it is available. If the async attribute is not present but the defer attribute is present, then the script is executed when the page has finished parsing. If neither attribute is present, then the script is fetched and executed immediately, before the user agent continues parsing the page.

https://www.w3.org/TR/2011/WD-html5-20110525/scripting-1.html#attr-script-async

Answer 3

Unfortunately defer is ignored when async specified, and async always have higher priority. (At least in Chrome. Honestly, did not tested in other browsers, but I think that result will be the same.)

And personally I think that it's very bad that defer is ignored. Let's imagine situation when we would like to have some JS initialized ASAP, even before loading of the page content. But we want this script initialized BEFORE the rest of scripts which requires it. It should be first in defer queue. But, unfortunately, this will not work:

<!-- we want "jQuery" ASAP and still in "defer" queue. But defer is ignored. -->
<script src="jquery.min.js" async defer></script>

<!-- this doesn't blocks the content and can wait the full page load, but requires "jQuery" -->
<script src="some_jquery_plugin.min.js" defer></script>

In this sample the "some_jquery_plugin.min.js" can be loaded and executed before the load of jQuery, and will fail. :(

So there is 2 ways to solve the problem: either use only defer directive, or merge all depending javascript files into single JS.

Answer 4

Yes, its valid HTML and it will work like expected.

Any W3C compliant browser will recognize the async attribute and treat the script properly, whereas legacy IE versions will recognize the defer attribute.

Since both attributes are boolean you don't have to assign any value.

Answer 5

Actually, there is a misconception in most comments here. People don´t seem to know that DEFER also loads the code in parallel, just like ASYNC. But it waits to execute after DOM loads (but before DOMContentLoaded executes), while ASYNC runs it immediately after loading, before DOM loads. Since BOTH ASYNC and DEFER load code in parallel, there would be no need to use both together (but to deal with legacy, maybe).