Automatic counter with stop and start buttons

<script src="angular.min.js"></script>
<script>
angular.module('CounterApp', [])
    .controller('CounterController', ['$scope', '$timeout', function($scope, $timeout) {
        var timer;
        $scope.counter = 0;
        $scope.stopCounter = function() {
            $timeout.cancel(timer);
            timer = undefined;
        };
        $scope.startCounter = function() {
            if (timer === undefined) {
                updateCounter();
            }
        };
        var updateCounter = function() {
            $scope.counter++;
            timer = $timeout(updateCounter, 1000);
        };
        updateCounter();
    }]);
</script>
<div ng-app="CounterApp">
   <div ng-controller="CounterController">
   {{counter}}
   <button ng-click="stopCounter()">Stop</button>
   <button ng-click="startCounter()">Start</button>
   </div>
</div>