Disable f12 in web page

<script language="JavaScript">
    document.onkeypress = function (event) {
        event = (event || window.event);
        if (event.keyCode == 123) {
           //alert('No F-12');
            return false;
        }
    }
    document.onmousedown = function (event) {
        event = (event || window.event);
        if (event.keyCode == 123) {
            //alert('No F-keys');
            return false;
        }
    }
document.onkeydown = function (event) {
        event = (event || window.event);
        if (event.keyCode == 123) {
            //alert('No F-keys');
            return false;
        }
    }
</script>