Ну, не знаю,  у меня работает:
<!DOCTYPE HTML>
<html>
 	<head>
 		<title>qwerty</title>
		<meta HTTP-EQUIV=Content-Type CONTENT="text/html; CHARSET=UTF-8">
	</head>
  	<body>
    	<script>
			function qwerty ()
				{
  					var field = document.getElementById("fieldName");
					alert(field.value);
  				return;
				}
    	</script>
    	<form>
			<input type="text" name="fieldName" id="fieldName" >
      		<input type="button" onclick="qwerty(fieldName.value)" value="Вывод" >
    	</form>
  	</body>
</html>