Which Javascript library?

[eluser]WolfgangA[/eluser]
I guess you mean something like autocomplete. You type in a word and get a list of words matching while you type.
If this is what you mean:
It is a simple usecase for AJAX / XHR combined with a database lookup.
You type or click, pass the content of your input field on each event via XHR to the server, execute a database query and send back the result via XHR to populate a widget like a combobox, menu or a simple div.
In terms of CI, you would call a controller/method, query a model and return the result in a view (that view can actually be very simple - even only JSON or XML).