Cannot set Angular scope variable from within ajax call

You forgot to initiate the digest cycle by calling $scope.$apply() after you modify the scope variable from non-angular context.

success: function (data, status, xhr) {
    $scope.req_result = "Post succeeded";
    $scope.$apply();
},
error: function (xhr) {
    $scope.req_result = "ERROR: " + xhr.responseText
    $scope.$apply();
}

Angular does the magical view updates by creating something called "watchers". When you do {{ someScopeVariable }}, Angular is creating watcher for that scope variable. Just as when you programmatically create a watcher by doing

$scope.$watch('someScopeVariable', function myListenerFunction() {
    // When someScopeVariable changes, do this
});

Watchers go hand-in-hand with something called digest cycle. From different places within Angular, e.g. $timeout callback, or $http.then() callback etc., Angular invokes the digest cycle from the $rootScope. This is basically going through all the watchers in the scope and if the value being watched has changed, invoke the listener function like the myListenerFunction above.

Important thing is Angular is triggering this digest cycle internally. So, when you use $.ajax() from jQuery, you are missing this. You change the scope variable that are being watched because you did

{{req_result}}

but, no watchers know about it. $scope.$apply() makes sure all the watchers know about this change by initiating a digest cycle from the root scope.

BUT

A better solution to your issue would be to use Angular's internal $http service which can be used to make XHR request and Angular automatically triggers digest cycle when you callback functions are invoked.

var req = {
            "RequestId": $scope.$parent.req_id,
            "RequestDate": new Date(),
            "RequestDetails": $scope.$parent.details
        };

$scope.req_result = "test";

$http
    .post('http://localhost:49358/api/myClasses/PostRequest', req)
    .then(
        function () {
            $scope.req_result = "Post succeeded";
        }, 
        function () {
            $scope.req_result = "ERROR";
        }
    );