add async to script tag code example

Example 1: async vs defer

Check this image for better understanding
//https://i.stack.imgur.com/wfL82.png

Async scripts are executed as soon as the script is loaded, so it doesn't guarantee the order of execution (a script you included at the end may execute before the first script file )
Defer scripts guarantees the order of execution in which they appear in the page.

Example 2: javascript async script

<script async src="script.js"></script>