element 对象：
<!DOCTYPE html>
<html>
<head>
<title>js22.html</title>
<script type="text/javascript">
function moveNext(object, index)
{
if (object.value.length == 4)
{
document.forms[0].elements[index+1].focus();
}
}
function showResult()
{
var f = document.forms[0];
var result = "";
for (var i = 0; i<4; i++)
{
result += f.elements[i].value;
}
alert(result);
}
</script>
</head>
<body onload="document.forms[0].element[0].focus();">
<form>
<input type = "text" size= "3" maxlength = "4" onkeyup="moveNext(this,0);">-
<input type = "text" size= "3" maxlength = "4" onkeyup="moveNext(this,1);">-
<input type = "text" size= "3" maxlength = "4" onkeyup="moveNext(this,2);">-
<input type = "text" size= "3" maxlength = "4" onkeyup="moveNext(this,3);">
<input type="button" value="显示" onclick="showResult();">
</form>
</body>
</html>
结果就是：
1. 输入完第一个 text field 之后，会自动跳转到下一个 text field.
2. 点击 显示按钮， 会弹出输入的所有 text
