What is the difference between async and defer in the script tag?

A normal <script src="..."> stops the browser from reading the HTML until the script is downloaded and run. async and defer both download the script in the background, but they run it at different times.

Point	async	defer
Download	In the background	In the background
Runs	As soon as downloaded, can interrupt the HTML	After the whole HTML is read
Order of scripts	Not guaranteed	Same as in the page
Use for	Independent scripts, like analytics	Scripts that need the page or each other

<script src="analytics.js" async></script>
<script src="app.js" defer></script>

For most of your own scripts, defer in the head is the best choice.

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