SharePoint Java Script does not work how to debug solution

This last week at a customer of mine we were building a custom solution with Javascript. As a few of you already know I made a JS Class which you can place in a document library. At the customer we place the scripts in a toplevel site of SharePoint Library. So what happened and what is the solution. We created a few ASPX. Webpart pages with content editors in there. We refrenced the code in de document library. And it seemed to work all fine for them working as Site Admin. But the client had different permissions viewer permissions only. But he did not see anything just a empty content editor. So I had to came in and help them a hand. I started debugging as administrator first because they told me they migrated to another farm and parts were still on the old farm. So I thought maybe it is cross site problem which could give lots of issues. But as a administrator I could see everything fine. So I started to look at the permissions of the viewers. The scripting permission was switched of turned that on. Still de same problem. Then I started fiddler to see which page where blocking [...]