Untitled

<!DOCTYPE HTML>
<html>
<head>
<title>slejw ssie</title>
<script>
function JK()
{
    var text = document.getElementById("xxx").value;
  if(text.length%8 == 0)
    KJ();
}
function KJ()
{
    alert("8 znaków");
}
</script>
</head>
<body>
<input type="text" id="xxx" OnInput="JK()"/>
</body>
</html>