Using $dirty in angularjs in order to check whenev

I was trying to check whenever my form is being edited by writing some fields of it. I read $dirty should work for that task but I can't figure out what I'm missing here:

<!DOCTYPE html>
<html lang="en">
<script src= "http://ajax.googleapis.com/ajax/libs/angularjs/1.3.14/angular.min.js"></script>
<body>

<div ng-app="myApp" ng-controller="formCtrl">
  <form name = "myForm" novalidate>
    First Name:<br>
    <input type="text" ng-model="user.firstName"><br>
    Last Name:<br>
    <input type="text" ng-model="user.lastName">
    <br><br>
    <button ng-click="reset()">RESET</button>
  </form>
  <p> is Form dirty? {{isDirty}}<p>
  <p>form = {{user }}</p>
  <p>master = {{master}}</p>
</div>

<script>
var app = angular.module('myApp', []);
app.controller('formCtrl', function($scope) {
    $scope.master = {firstName:"John", lastName:"Doe"};
    $scope.reset = function() {
        $scope.user = angular.copy($scope.master);
    };
    $scope.reset();
    $scope.isDirty = $scope.myForm.$dirty;
});
</script>

</body>
</html>

I'm trying to make the flag isDirty to true whenever the user modifies the form. Thanks

You are missing name attributes in your form fields which are not enabling form validation for those field. You need to add unique name for each field so that it will get add those field in myForm object

Markup

  <form name="myForm" novalidate>
    First Name:<br>
    <input type="text" name="firstName" ng-model="user.firstName"><br>
    Last Name:<br>
    <input type="text" name="lastName" ng-model="user.lastName">
    <br><br>
    <button ng-click="reset()">RESET</button>
  </form>

Also you are accessing myForm object which is nothing but form object, I won't be available until DOM get rendered, $scope.myForm will be simply undefined at the time of controller initilization, If you really want to access $scope.myForm from controller then you need to put that code in $timeout that will run $timeout function code in next digest cycle.

  $timeout(function(){
    $scope.isDirty = $scope.myForm.$dirty;
  });

Update

There is no need to maintain a separate isDirty flag (this would require to change the separate isDirty flag to reflect any changes in myForm.$dirty flag.) Instead I suggest you use $scope.myForm.$dirty directly as a flag. So use the expression myForm.$dirty, and this flag will change as form gets dirty.

Working Plunkr