判断两个文本框中只输一个的实例：
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
<title>标题页</title>
<SCRIPT LANGUAGE="JavaScript">
function checkFields()
{
name = document.submitform.name.value; //获取姓名
email = document.submitform.email.value; //获取邮箱
if ((name == "") && (email == "") ) { //如果两者都为空
alert("请输入姓名和邮箱！");
return false;
}
else
return true;
}
</script>
</head>
<body>
<form name=submitform onSubmit="return checkFields()">
名字和邮箱只需要输入一个即可<br>
<br>
<table border=0>
<tr>
<td align=center>名字</td><td> </td>
<td align=center>Email</td></tr><tr>
<td align=center><input type=text name=name value="" onFocus="document.submitform.email.value='';" size=10></td>
<td align=center>或</td>
<td align=center><input type=text name=email value="" onFocus="document.submitform.name.value='';" size=10></td>
</tr>
<tr>
<td colspan=3 align=center><input type=submit value="提交"></td>
</tr>
</table>
</form>
</body>
</html>
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