FileSystem problem - cordova Ionic Application

I test with firefox or chrome.
I do:> Ionic serve

I just tested:

 Error: cordova is not defined

code :

app.controller('AddCtrl', function($scope, $cordovaFile, $location, $ionicPlatform) {

      $ionicPlatform.ready(function() {
        $scope.addAss = function () {

      $cordovaFile.createFile(cordova.file.dataDirectory, "new_file.txt", true)
        .then(function (success) {
          alert('ok1');
        }, function (error) {
          alert('ok2');
        });

      };

      });
});