String str = "Hello";
//not working var str = "<%=str%>";
If you think that the java script is written inside the JSP then its possible.
But you are providing the JS file separately then you can make a global variable and that variable will be available everywhere.
Your not going to be able to write to the external js file. However if you put
var s = "<%=str%>";in your jsp s will become a global variable which can be used within your external js file because s will have a global scope. So in a nutshell, you can't modify the actual js file, but you can add a variable with global scope and reference that variable within your external js script.