Angular:

http://angularjs.org/

HTML

<div ng-controller="MyCtrl">
  <form id="registerForm" ng-submit="create(email, username, password)">
    <input type="text" name="email" placeholder="Email" ng-model="email">
    <input type="text" name="username" placeholder="Username" ng-model="username">
    <input type="password" name="password" placeholder="Password" ng-model="password">

    <input type="submit" value="register" class="button" ng-show="!loading">
    <input type="submit" value="processing" class="button" disabled ng-show="loading">
</form>
</div>

JavaScript

var app = angular.module('myApp',[]);


function MyCtrl($scope) {
    $scope.$watch('email', function(value) {
        $scope.username = value
    });
}