I just figured out how to obtain js content assist in Eclipse JavaScript editor (without Aptana plugins):
If the project type doesn't natively contain JavaScript Support: open Web (or JavaScript) perspective, right-click on the project and select Web Development Support > Add JavaScript Support (this won't hurt if JavaScript support is already present)
then right-click JavaScript Support within the project and select Properties, in the JavaScript section go to JavaScript library and then select Source tab: here you can add folders and files to be scanned by content assist for the current project
In Aptana Studio (as an eclipse plugin but I suppose the standalone version is almost the same): open any js or html file, show References window (or open Aptana perspective in eclipse) and drag-drop js files you want to add to JavaScript scope (it is possible to build and activate different scope profiles with different JavaScript files and resources: just click add profile in the window toolbar)
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