Get Sharepoint Control ID using Javascript in Sharepoint 2010

<asp:Content ContentPlaceHolderId="PlaceHolderMain" runat="server">

//find a controlid like textbox or dropdownlist

function getControlID(tagName, identifier, title) {

            var len = identifier.length;

            var tags = document.getElementsByTagName(tagName);

            for (var i = 0; i < tags.length; i++) {

                var tempString = tags[i].id;

                if (tempString.match(title)) {

                    return tags[i].id;

                }

            }

            return null;

        }

function Caller()

{

//you can call like this

  var dropdownid = getControlID("select", "Choice", "Content Type");

}

  _spBodyOnLoadFunctionNames.push("Caller");

</asp:Content>