async: Use <script async src=”.js”></script> to load js file in html page
<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
defer: Use <script defer src=”.js”></script> to load js file in html page
How is a html page is displayed by browser?
However, once a js file is loaded, it will start to run immediately. Browser will pause to render html when js file is run.
You should notice: