Hi,
First create one html or php file then copy and paste below code.

<script language="javascript">
var strValue = "How are you...";
</script>

Then create another file called demo.js in that file copy and paste below code
alert("Hi, "+strValue);

Now include javascript in the html or php file like below
<script language="javascript">
var strValue = "How are you...";
</script>
<script language="javascript" src="demo.js"></script>

We declare global so we can access it anywhere.