原生ajax方法

function ajax(options){
    options = options || {};
    var method = (options.type || "GET").toUpperCase(),
        url = options.url,
        queryString = null;
    if(!url)
        return;
    if(options.data){
        queryString = [];
        for(var attr in options.data){
        queryString.push(attr + "=" +options.data[attr]);
        }
        queryString = queryString.join("&");
    }
    if(method === "GET" && queryString){
        url += "?"+queryString;
        queryString = "";
    }
    var xhr = new XMLHttpRequest();
    xhr.open(method,url,true);
    if(method === "POST")
        xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xhr.send(queryString);
    xhr.onreadystatechange = function(){
        if(xhr.readyState === 4){
        if(xhr.status === 200){
            var data = xhr.responseText;
            if(options.dataType === "json")
            data = JSON.parse(data);
            options.success && options.success(data);
        }else{
            options.error && options.error(xhr.status);
        }
        }
    }
}