Why are elements null in jquery, yet exist with document.getelementbyid() - asp.net

Question

I"m trying to attached some jquery to checkboxes in a gridview, using document.ready:
$(document).ready(function()
{
var chkBox= document.getElementById("gvTimeSheet_ctl01_chkAll1");
//I can alert chkBox.id, element exists
var name = $("input[name='gvTimeSheet$ctl01$chkAll1']");
//Here, when I alert the id, I get a null
var ID = $("#gvTimeSheet_ctl01_chkAll1");
//Here, when I alert the id, I get a null
var withClass = $(".chkAll1Class");
//Here, when I alert the id, I get a null
var withClass2 = $(".Bill1");
//Here, when I alert the id, I get a null
//This line causes the browswer to crash and gives me the following error
//Microsoft JScript runtime error: 'null' is null or not an object
$("#gvTimeSheet_ctl01_chkAll1").click(function()
{
var checked_status = this.checked;
$("input[class=Bill1]").each(function()
{
this.checked = checked_status;
});
});
});*/
So, why are any attempts at finding an object null in jquery, yet exist in regular javascript within the same method? What am I missing here. I have the jquery js files brought in in a script tag directly above this method. I just can't seem to find any objects on this page with jquery. On other pages, I can.

Answer 1

Objects that result from a jQuery selector are actually wrappers around a DOM object, so you don't access it the same as a DOM object.
If you're alerting just "name.id", from your first example above, there won't be any such property on the jQuery wrapper. Try alerting your ID as follows:
alert(name.attr("id"));

Answer 2

var val = $("input:radio[name$='rdoselect']:checked").val();
if (val == 1) {
$('[id$=divDate]').attr('disabled', true);
}else {
$('[id$=divDate]').attr('disabled', false);
}

Answer 3

var ID = $("#gvTimeSheet_ctl01_chkAll1");
This returns a jQuery object, not an ID. ID.id would also be undefined. To get the ID, you need:
var ID = $("#gvTimeSheet_ctl01_chkAll1").attr("id");

Answer 4

Does the page you're adding this code to already include the Prototype JavaScript library?
jQuery's "$" method never returns null, so this shouldn't be a problem:
// This line causes the browswer to crash and gives me the following error
// Microsoft JScript runtime error: 'null' is null or not an object
$("#gvTimeSheet_ctl01_chkAll1").click(function() { .... });
All the comments about needing to use .attr('id') still stand (though I prefer $('#whatever')[0].id myself.)