<script src="test.js" async></script>
If async attribute is present in script tag then script is executed asynchronously.
async attribute is a Boolean attribute.
What is the difference between async and defer?
If async is present the file gets downloaded asynchronously and then executed as soon as it’s downloaded.
If defer is present file is downloaded asynchronously but its only executed when document is parsed completely and scripts are executed in the order they are present in the code.