Check out example codes for "link javascript html". It will help you in understanding the concepts better.
Code Example 1
<script type="text/javascript" src="monscript.js"></script>
Code Example 2
<script src="index.js"> </script>
Code Example 3
<script src="index.js"></script>
Code Example 4
<script src = "index.js"></script>
Code Example 5
<script type="text/javascript"
src="file.js">
</script>
Code Example 6
<script src="Your-JavaScript-File.js"></script> <!--If JS file is in same folder with HTML file-->
Learn ReactJs, React Native from akashmittal.com