Question: Anybody knows a Editor where i can develop my JS directly in the Browser with a js based Editor? 

It would be nice if the code completion is customizable so that you can add own objects and methods, possibly documentation for each method / object.
Code completion is not required for the objects that come with javascript (like window) as we're using Mozilla's JS Framework Rhino.

Thanks in advance

Best Regards
Flexo

Answer: 
The only browser based JS/page editor that I know of is Mozilla's Bespin, <https://bespin.mozillalabs.com> , but I have no idea if they do code-completion.

There is a JS tool called JSLint from the Crockenator (Douglas Crockford) which you might find helpful, <http://www.jslint.com>. No code-completion, but it lets you write good JavaScript.

My two cents.
I just stumbled across a JavaScript implementation of vi. But that doesn't do autocompletion.

[this one](http://ajaxian.com/archives/byteplug-experimental-online-javascript-editor) claims to do so, though. I know nothing else about it, though.