ajax中status返回结果是0是怎么回事？

Question

window.onload=function(){ var request=new XMLHttpRequest(); request.open("GET","http://study.163.com /webDev/couresByCategory.htm",true); request.send(); request.onreadystatechange=function(){ if (request.readyState===4&&request.status===200) { var data=JSON.parse(request.responseText); document.getElementById("name").innerHTML=data.name; }else{ alert("发生错误"+request.status); } } }