factory.js

function User(attrs) {
  for(var name in attrs) {
    this[name] = attrs[name];
  }
}

User.build = function(attrs) {
  if(arguments.length === 2) {
    attrs = {
      name: arguments[0],
      email: arguments[1]
    }
  }

  return new User(attrs);
}

var user1 = User.build({
  name: 'Mauricio',
  email: 'm.soares'
});

var user2 = User.build('Mauricio', 'email2');