In Eclipse content assist is usually implemented by some class that implements IContentAssistProcessor
(JavaDoc, example)
How to implement the s
You need to implement the completion proposal extension point of the JSDT editor. In there you have to specify a class implementing the necessary interface, but it is a different one than the very generic IContentAssistProcessor.
Depending on your use case you may also want to have a look at the related quickAssist and quickFix extension points.