What is a call back function?
What is a call-back function?
Rails, Tekla Open API, AutoDesk .NET
What is a call-back function?
This used to confuse the life out of me.

I have had the glorious pleasure of reading WriteBook - mainly for the benefit of understanding how the creators of Basecamp writes code.
Here is a snippet from 37 Signal’s Writebook code base. I’ve extracted it as a quiz for you: