We recommend JSDT for development of Molybdenum inside Eclipse as it gives you type inference and thus basic code completion and validation support. More infos here http://www.ibm.com/developerworks/library/os-eclipse-jsdt/index.html...
For proper inference, use JsDoc Tag Reference
JSDT has become part of Eclipse Web Tools Platform Source Editing Project, download latest stuff from there.
Some issues and further infos:
