My employer has asked me how to mix the functionality of a textfield input with a select input, “kinda like gmail does when you start typing the address of someone you’ve already written to, it allows you to choose the person and also to keep writing if it isn’t in the database”.
How is this done? I imagine there’s a javascript/jquery library to implement this.
Yes, there’s a whole lot of such plugins. These are probably two most popular (most often asked on SO) and customizable:
http://bassistance.de/jquery-plugins/jquery-plugin-autocomplete/ (demo here)
http://jqueryui.com/demos/autocomplete