Eclipse JavaScript Editor: content assist for js files, autocompletion

imo, Aptana is the best eclipse plugin for js editing. It includes support for many of the major libraries like jQuery, yui, dojo, etc.

Spket, however is also good. Though mostly if you do firefox extension development (getting a little out of date though)

The built in js editor is terrible. it claims there are errors all over the place even when there clearly are not.

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

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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)