title: What is a Callback API?
body: <p>I know what a Callback is, and I know what an API is. But what, exactly, defines a callback API? How is it implemented? How does it differ from REST APIs for example? To narrow it down, let's say callback APIs in NodeJS.</p>
  
  <p>And are we talking about the client-side API, or the server-side API?</p>
  
  <p>For a client-side API I could guess something like this:</p>
  
  <ol>
  <li>You make an API call to some url.</li>
  <li>You receive a response</li>
  <li>A callback function is called with the response as its arguments</li>
  </ol>
  
  <p>This callback function is implemented in client-side, and you just pass the callback to the service function which actually makes the API call.</p>
  
  <p>What about server side callback API?</p>