jQuery's $() function always returns 'undefined' with AJAX

Answer 1

One thing you could try is using Firebug, or some other DOM inspector and check the actual element IDs that are being generated by ASP.NET before and after your AJAX call and see if they are the same.

Answer 2

You're just missing your id selector syntax. Try:

$('#<%= txtRcaNotes.ClientID %>').hide(); 
alert($('#<%= txtRcaNotes.ClientID %>').attr('id'));

Note the addition "#" prepended before each selector.