gistfile1.js

function signup(successCallback) {
  $.ajax({
    url: 'http://signup.com',
    type: 'post',
    data: {username: "butt"},
    success: function(response) {
      successCallback(response.user);
    },
    error: function() {
      alert('well...');
    }
  });
}

$('#button').bind("click", function() {
  signup(function(newUser) {
     alert('welcome, ' + newUser.username);
  });
});