When your upload is complete, click on Library under the Media menu. This gives you a list of all files that you have uploaded to your website. On the top of the list should be your js file that you uploaded earlier. You will also notice that other media files (image files) are represented by their thumbnails while your js file is represented by some default wordpress icon. Click on this icon. You'll be taken to the Edit Media page. You don't need to edit anything but on the right you should see File URL. Under this there is a url that points to the js file. Use this link on your script tags. Thats all! Done!