js中in这个关键字是干什么的啊

<SCRIPT LANGUAGE="JavaScript">
var o=new Object();;
o.name="zkj";
o.age=25;
o.bir=new Date();;
for(var key in o);{
document.writeln(key+':'+o[key]+'</br>');;
}
这个var key in o  

"in"有什么用啊,
</SCRIPT>

那些书有讲到这些知识啊,我的那本感觉讲的知识太少了.

ECMAScript 关键字 http://www.w3school.com.cn/js/as_js_keywords.aspbreakcasecatchcontinuedefaultdeletedoelsefinallyforfunctionifininstanceofnewreturnswitchthisthrowtrytypeofvarvoidwhilewithECMAScript 保留字 http://www.w3school.com.cn/js/as_js_reservedwords.aspabstractbooleanbytecharclassconstdebuggerdoubleenumexportextendsfinalfloatgotoimplementsimportintinterfacelongnativepackageprivateprotectedpublicshortstaticsupersynchronizedthrowstransientvolatile原始类型primitive typeUndefined、Null、Boolean、Number 和 StringDOM对象Window Navigator Screen History Location Document