JavaScript: formatar data

<script language="JavaScript" type="text/JavaScript">
function formatar(src, mask)
{
 
    // permitindo alguns caracteres importantes
    switch(event.keyCode)
    {
        case 8:     // backspace
        case 9:     // tab
        case 37:    // left arrow
        case 39:    // right arrow
        case 46:    // delete
        event.returnValue = true;
        return;
    }
 
    var i = src.value.length;
    var saida = mask.substring(0,1);
    var texto = mask.substring(i)
    if (texto.substring(0,1) != saida)
    {
        src.value += texto.substring(0,1);
    }
}
</script>