how to do a javascript file code example

Example 1: calling javascript file in html

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

Example 2: how to create js code

<!--just open a file with the ending of ".js"
example:  myFirstJS.js

an other option is to write this sentence on HTML document:
-->

<script type="text/javascript">
//write here your code
</script>