How do you call a controller method from another controller?

[Reil G]

What is the best practice for this when building a REST API? 

Say I have a search controller with methods I'd like to be able to use within other controllers? How would I want to go about that? 

I'm still relatively new to developing in javascript, so I really don't know how to do this. Heck, I don't even know where to start. 

Googling it doesn't seem to yield anything.