When I click on this checkbox (inside the Modal form)
<input id="selectallcreate" type="checkbox" value="1" name="selectallcreate">
It will always return False using this code
if ($('#selectallcreate').is(':checked') == true){};
Also this code will work perfectly on any page that are NOT modal. Other then that the modal form operates perfectly.
Be sure in your DOM that there’s only ONE instance of id
selectallcreate. You might be surprised to find that you have two. It’s happened to me. I’d say, try locating the checkbox by$('QueryToGetDialogDiv').find("#selectallcreate"), so you can be sure you’re grabbing the correct div.