Facendo così funziona.

<html>
<head>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script language="JavaScript">
function controlla()
{
if (document.pippo.pluto.value == "")
alert('compila');
}

</script>
</head>

<body>
<form name="pippo" onSubmit="return controlla()">
<textarea name="pluto">
</textarea>
<input type="submit" value="invia">
</form>
</body>
</html>