why the variable URL is undefined?

why the variable URL is undefined?

I have a problem with a variable that take the value undefined.
In the file Create.cshtml i have:
@using (Html.BeginForm("Create", "Enrollment", FormMethod.Post, new
{
id = "YearCourseFormId",
data_courseListAction = @Url.Action("CourseList")
})) {
@Html.AntiForgeryToken()
@Html.ValidationSummary(true)
<fieldset>
<legend>Enrollment</legend>
<div class="editor-label">
@Html.LabelFor(model => model.StudentId, "Student")
</div>
<div class="editor-field">
@Html.DropDownList("StudentId", ViewBag.Student as SelectList,
String.Empty, new { @class = "chosen-select", data_placeholder =
"Please Select a Student...",style="width:350px;" })
@Html.ValidationMessageFor(model => model.StudentId)
</div>
<div class="editor-label">
@Html.LabelFor(model => model.Course.YearId, "Year")
</div>
<div class="editor-field">
@Html.DropDownList("YearId", ViewBag.Year as SelectList,
String.Empty, new { @class = "chosen-select", data_placeholder =
"Please Select a Year...",style="width:250px;" })
@Html.ValidationMessageFor(model => model.Course.YearId)
</div>
<div class="editor-label">
@Html.LabelFor(model => model.CourseId, "Course")
</div>
<div class="editor-field">
@Html.DropDownList("CourseId", String.Empty)
@Html.ValidationMessageFor(model => model.CourseId)
</div>
<p>
<input type="submit" value="Create" />
</p>
</fieldset>
}
In the Jquery File i have:
$(function () {
$('#YearId').change(function () {
var URL = $('#YearCourseFormId').data('courseListAction');
alert(URL);
$.getJSON(URL + '/' + $('#YearId').val(), function (data) {
var selectList = $("#CourseId");
selectList.empty();
var option = $('<option>');
selectList.append(option);
$.each(data, function (index, optionData) {
option =
$('<option>').text(optionData.Text).val(optionData.Value);
selectList.append(option);
});
});
});
});
In this part alert(URL);the result is undefined.
In the EnrollmentController.cs i have:
public ActionResult CourseList(string ID)
{
int Year = int.Parse(ID);
var courses = from s in db.Courses
where s.YearId == Year
select s;
if (HttpContext.Request.IsAjaxRequest())
return Json(new SelectList(
courses.ToArray(),
"Id",
"CourseName")
, JsonRequestBehavior.AllowGet);
return RedirectToAction("Index");
}
But this method is not called due to error.
What is the problem?