title: What does stub.callsArg(index) from Sinon.JS do?
body: <p>Seriously, I can't figure this out. The documentation gives us:</p>
  
  <p><strong>stub.callsArg(index)</strong> - Causes the stub to call the argument at the provided index as a callback function. stub.callsArg(0); causes the stub to call the first argument as a callback.</p>
  
  <p>However, I've got no idea where this list of arguments to be indexed into is. Maybe I just don't understand what a stub is...</p>