MVC 5 JsonResult returns html? - asp.net

Question

I have been following this tutorial https://www.youtube.com/watch?v=c_MELPfxJug regarding ajax and JsonResult in HomeController
I did the tutorial, however for some reason the controller is returning Html and not json
I did not change one line of code, but it's failing with parseError on the javascript side.
when i look at the response i see an html page, not a json object.
Controller code:
public JsonResult DoubleValue(int? Value)
{
if (!Request.IsAjaxRequest() || !Value.HasValue)
{ return null; }
else
{
int DoubleValue = Value.Value * 2;
var ret = new JsonResult
{
Data =
new { DoubleValue = DoubleValue }
};
return ret;
}
}
cshtml:
#using (Html.BeginForm())
{
#Html.TextBox("txtAmount",0)
<button id="btnDoubleValue">DoubleIT</button>
<div id="lblMessage"></div>
}
#section Scripts{
<script type="text/javascript">
$(function () {
$('#btnDoubleValue').on('click', function() {
$.ajax({
type: 'POST',
url: '#Html.Action("DoubleValue")',
data: { 'Value': $('#txtAmount').val() },
datatype: 'json',
cache: 'false'
}).success(function (data) {
var t = data;
$('#txtAmount').val(data.DoubleValue);
}).error(function (x, o, e) {
$('#lblMessage').html('error was found: ' );
});
return false;
})
});
</script>
}

Answer 1

found the error
I was using Html.Action and not Url.Action -> just human error I suppose
from the reference:
Html.Action - returns the result as an HTML string.
It works now
$.ajax({
type: 'POST',
url: '#Url.Action("DoubleValue")', //<--- Url.Action
data: { 'Value': $('#txtAmount').val() },
datatype: 'json',
cache: 'false'

Answer 2

I guess this must be the default error page, you are probably getting a 500 response and you must use the Network tab of your browser to see the real problem.
In your browser open developer tools using F12 key and navigate to Network tab.
Make the appropriate actions to do the ajax request (click on that button)
Click on the request row
Navigate to Response tab.
From there you can watch the real request your ajax does and the response from the server.