Using the async attribute on a script tag, makes the script load asynchronously and executes it then.
<script src="example_script.js" async></script>
Using the defer attribute on a script tag, makes the script load asynchronously and then wait until the whole page is finished parsing, for the script to execute. This attribute shouldn’t be used if the scr attribute on the script tag is absent as this has no effect on inline scripts.
<script src="example_script.js" defer></script>