Hello Marcel,
You can allow specific users to read/write specific folders by adding them in the "security" tab of the folder properties, as described here. I'm not sure this method will work though. So to be sure, what you can do is add the ADMINISTRATION named right to that user. The user won't be added to the administrator group. For example, he will then be able to access the Administration > Configuration > JavaScript codes folder and create new elements.
Also, if you want to be able to create JS code in a workflow, you will need the Workflow named right.
Hope this helps,
Florent.