I am getting exception in document.ready as $ is undefined, Why?
 if I removed document .ready function other jquery code working fine . following is my code 

```
 <script type="text/javascript">
        $(document).ready(function () { alert('Hi'); });
        function LoginTypeChanged() {
            if ($('#loginType option:selected').val() === "1") {
                $("#txtUserName").prop('disabled', false);
                $("#txtPassword").prop('disabled', false);
                $("#txtDomain").prop('disabled', false);
            }
            else if ($('#loginType option:selected').val() === "0") {
                $("#txtUserName").val("");
                $("#txtPassword").val("");
                $("#txtDomain").val("");
                $("#txtUserName").prop('disabled', true);
                $("#txtPassword").prop('disabled', true);
                $("#txtDomain").prop('disabled', true);

            }
        }
</script>

```