Loading a JavaScript file using both defer and async

Question

What would be the loading order of these two JavaScripts?

<script src="a.js">
<script src="b.js" defer async>

What happens to b.js that has both defer and async?

"If you specify both, async takes precedence on modern browsers, while older browsers that support defer but not async will fallback to defer." Source: flaviocopes.com/javascript-async-defer — Mateus Wolkmer
– Mateus Wolkmer, Commented Jul 29, 2019 at 20:02

Daniel Richter · Accepted Answer · 2019-07-30 00:52:44Z

2

As specified here: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script

Async is a Boolean attribute indicating that the browser should, if possible, load the script asynchronously.

While Defer is a Boolean attribute that indicates to a browser that the script is meant to be executed after the document has been parsed, but before firing DOMContentLoaded.

So in your case, b.js would load after a.js

answered Jul 30, 2019 at 0:52

Daniel Richter

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