Callbacks (or Sinks) are functions that return nothing. "Nothing" is most often a null or undefined, but sometimes it's just a non-data result (setTimeout return value). Also, with dynamic typing, you can choose to ignore a function result and use a pretty common function as a callback. That's why we introduce the term of "nothing" here.

Which of the next functions are real callbacks?

  1. Case
  1. Case
  1. Case