app.post('/login', function (req, res) {
    var username = req.body.username; // etc...
    if (username === "George" && password === "cat") {
        res.json({message : "success"});
    } else {
        res.json({message : "fail"});
    }
});

app.post('/login', function (req, res) {
    var username = req.body.username; // etc...
    if (username === "George" && password === "cat") {
        res.json({status: 200, message : "success"});
    }
    else {
        res.json({status: 401, message : "fail"});
    }
});

$.post( "/login", function(loginData) {

})
  .done(function(data) {
      if(data.status === 200) {
          //logged in
      } else {
          //logged out
      }
  })
  .fail(function() {
      //error (bad connection)
});

$('#some-button').on('click', function() {
  //lets go make a request with some ajax call

  // lets say ajax call fails and responds with something like this:
  var error = {
    message: 'Could not retrieve data.',
    code: 403
  };

  //We can build a string of html, add the error fields to the html, and add that html to our document like so.
  var errorHtml = "<div class='error'><h3>Sorry, an error has occured.</h3><p>Message: _message</p><p>Code: _code</p></div>"
  .replace('_message', error.message)
  .replace('_code', error.code)

  // Add to document
  $('#error').html(errorHtml)
});