title: What is the attribute "done" in NodeJS?
body: <p>I'm coding the local login in NodeJS following this tutorial:</p>
  
  <p><a href="https://scotch.io/tutorials/easy-node-authentication-setup-and-local" rel="noreferrer">https://scotch.io/tutorials/easy-node-authentication-setup-and-local</a></p>
  
  <p>In the file config/passport.js</p>
  
  <pre><code>function(req, email, password, done){
      process.nextTick(function(){
          User.findOne({'local.email' :   email}, function(err, user){
              if(err)
                  return done(err);
              if (user){
                  return done(null, false, req.flash('signupMessage', 'message'));
              }
  </code></pre>
  
  <p>I'm rookie in NodeJS and Javascript, and I don't understand how a value like "done" can be a function (return done(err)). Is any system function?</p>
  
  <p>Thanks a lot!</p>