巧用恒等符号实现检验变量有没有定义

巧用恒等符号实现检验变量有没有定义

<html>

<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<meta name="GENERATOR" content="Microsoft FrontPage 4.0">
<meta name="ProgId" content="FrontPage.Editor.Document">
<title>function IsDefined</title>
</head>

<script>
function IsDefined(obj)
{
if(
"undefined"!==obj&&
("undefined"==obj+"")
)
return false;
return true;
}
alert(IsDefined(document.sss));
alert(IsDefined("s"));
alert(IsDefined("undefined"));
alert(IsDefined(document));
alert(IsDefined(111222));
alert(IsDefined(document.oncontexemenu));
</script>
</html>