XMLHttpRequest之onreadystatechange

function doReq(url, fn){
    var req = new XMLHttpRequest();
    req.onreadystatechange = function(){
        if(this.readyState == XMLHttpRequest.DONE){
            return fn(this);
        }
    }
    req.open('GET', url, false);
    req.send();
}
var res;
function myreq(xhttp){
    try{
        switch(xhttp.status){
            case 200:
                res = xhttp.responseText;
                break;
            case 404:
                throw new Error('404');
                break;
            case 500:
                throw new Error('500');
                break;
            default:
                throw new Error(xhttp.status);
                break;
        }
    }catch(e){
        res = e.message;
    }
}
doReq('https://www.w3school.com.cn/demo/js/ajax_info.txt', myreq);
console.log(res);