how to get Session value in external javascript file

You cannot use server side expressions in your external javascript files.
You could declare a global variable in your aspx page that will hold the
value which will be used in your javascript file:

in front page :

<script type="text/javascript">
var sessionexternal= '<%= Session["sessionexterna"] %>';
</script>


and then you could use sessionexternal variable in your external javascript file function.