//Include this script into your externatl master js file.
function $$(id, context) {
    var el = $("#" + id, context);
    if (el.length < 1)
    el = $("[id$=_" + id + "]", context);
    return el;
}
        

// Hence  we can easilly use $$ function in order to select and asp.net control.
   var username = $$("UserName");
   var password = $$("Password");

http://www.west-wind.com/weblog/posts/42319.aspx

// Taken from  :
// http://www.west-wind.com/weblog/posts/42319.aspx

Easy JQuery Asp.Net Control Selection

jquery, aspnet

jQuery