app.use('/some-end-point', function(req, res, next) {
  // some really cool code happens here <<<<
  // then an async call below. Notice the callback being passed into the save method
  User.save(function (err) {
    // the save method must call the database. Lots of things can go wrong here, so if the driver that connects to the database detects something bad it will call this function that we pass into it with the error it detects. We now need to be responsible campers and deal with this error.
    if (err) {
      // if we get to this point that means `err` exists and we need to end this function and call the callback.
      return next(err);
    }
    // because of the return in the conditional above, if we reach the code below that means the database returned with no errors. Now we can proceed.
   res.send('stuff happend, saved user, end of line');
   // it is important to end the route handler in two ways. Either by calling `next` with or without an err argument. This will tell express to look for the next route handler. The other way is to send something down to the user using a `res` method. `res` here is short for response meaning the response to the user. Most of the time you will be using `res.render` but you could also call `res.end` to not send the user anything other than a message that says the request to the server has ended. 
  });
});