I have a Java EE project which contains some JavaScript files. I followed the below option to get content assist for Javascript files.
Right click the project -> Configure -> Convert to JavaScript Project
(only .project and .settings folder of the project got modified)
No, the content assist is already builtin and is triggered on files with .js extension. To do a quick test, enter for and then Ctrl+Space, you should get a choice list with various loop templates. Configuration can be done by Window > Preferences > JavaScript.
The default JavaScript content assist is however pretty limited in capabilities as opposed to the Java content assist in .java files. If you want better content assist for JS (and HTML/CSS/XML files), consider installing the Aptana plugin.