<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>无标题文档</title> <script type="text/javascript"> function log(o){ console.log(o); } function lognull(o){ log(o==undefined) log(o==null) log(typeof o) } function notHave(o){ if(o==undefined){return true;} if(o==null){return true;} // if((typeof o)==undefined){return true;} return false; } var a={aa:‘bb‘} var c=null; lognull(a.ff)//未定义子元素不会报错 lognull(c) //以下，未定义顶级元素会报错 lognull(d) if(dd){ //Uncaught ReferenceError: dd is not defined } </script> </head> <body> </body> </html>
原文：http://www.cnblogs.com/stit/p/6367785.html
