Admin will be able to edit any js from admin panel in online. Admin not required to change file permission.
Quite simple, Download the module and simply put into your_drupal_path/sites/all/modules and install from your admin panel.
You are almost done. Please download latest release of codemirror from http://codemirror.net. After downloading codemirror extract into your drupal installation folder in following path sites/all/libraries/ and rename it as codemirror. If your library path looks like sites/all/libraries/codemirror that's mean you are ready.
Now you will have nice editor.
After successful installation choose permission for this module. Please browse /admin/people/permissions and set "Full HTML" for suitable user.
If everything is ready then go to following path Admin >> Configuration >> Jstool
Make sure you need write permission in your js file to edit.
1. Libraries API (http://drupal.org/project/libraries/)