说明
with 语句可以方便地用来引用某个特定对象中已有的属性，但是不能用来给对象添加属性。要给对象创建新的属性，
必须明确地引用该对象
示例
<html>
<head>
<script type="text/javascript">
function validate_email(field,alerttxt){
with (field){
apos=value.indexOf("@")
dotpos=value.lastIndexOf(".")
if (apos<1||dotpos-apos<2) {
alert(alerttxt);return false
}else {
return true
}
}
}
function validate_form(thisform){
with (thisform){
if (validate_email(email,"Not a valid e-mail address!")==false){
email.focus();
return false
}
}
}
</script>
</head>
<body>
<form action="submitpage.htm"onsubmit="return validate_form(thisd);" method="post">
Email: <input type="text" name="email" size="30">
<input type="submit" value="Submit">
</form>
</body>
</html>
