Check out example codes for "how to add script in html head". It will help you in understanding the concepts better.

Code Example 1

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

<!--just add defer attribute so script will execute at last-->

Code Example 2

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

Code Example 3

<script type="text/javascript" 
         src="file.js">  
</script>

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