<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function valida(){
var x=document.prueba.length;
var i=0;
for (i=0; i<x; i++){
if (document.prueba(i).value==''){
alert ('error');
break;
}
}
}
//-->
</script>
</head>
<body>
<form name="prueba">
<input type='text' name="obj1">
<input type='text' name="obj2">
<input type='text' name="obj3">
<button name="obj4" onclick='valida()'>pruebame</button>
</form>
</body>
</html>