title: What is the difference between Jest Mock functions and Sinon spies
body: <p>I am mocking a function with Jest and the documentation says they are really 'spies'. I have also seen the use of spies in SinonJS but I could find no clear difference between the two. If they are serving the same purpose, is there any reason to choose one over the other?</p>
  
  <p><a href="http://facebook.github.io/jest/docs/mock-function-api.html#mockfnmockimplementationfn" rel="noreferrer">Jest Mock Functions</a></p>
  
  <p><a href="http://sinonjs.org/" rel="noreferrer">SinonJS</a></p>