Concat Ajax成功响应

Question

Here is my ajax request success function

function(res){
   var proposedValue='res.'+ res.ProposedFor;
   $('#MyDynamicDropdown').val(proposedValue);
}

Here, proposedValue does return

res.AreaId

But it doesn't set my value in the dropdown. If I use

 $('#MyDynamicDropdown').val(res.AreaId);

That works fine.But I can't use this because I don't know what has been propose for. Thanks hope you understand my issue.