How to get the focus beside the last letter entered on my textbox

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C#

ASP.NET

Javascript

script

Here's the javasript I'm using:

<script type="text/javascript">
function setCursorPosition(elem, pos) {

        if (elem != null) {
            if (elem.createTextRange) {
                var range = elem.createTextRange();
                range.move('character', caretPos);
                range.select();
            }
            else {
                if (elem.selectionStart) {
                    elem.focus();
                    elem.setSelectionRange(caretPos, caretPos);
                }
                else
                    elem.focus();
            }
        }
    }
</script>

Here's the code on the button that will trigger the viewing of data on the textbox:

<asp:Button ID="btnEdit" class="buttonsMainteUN"  runat="server" Text="Edit" 
            ToolTip="Edit user account details" PostBackUrl="~/EditUser.aspx" OnClientCilck="var elem = document.getElementById('txtLastName');
setCursorPosition(elem,elem.value.length);"/>