函数封装：

1.封装一个辅助函数，把传进来的对象拼接成url的字符串

function toData(obj){
    if (obj == null){
        return obj;
    }
    var arr = [];
    for (var i in obj){
        var str = i+"="+obj[i];
        arr.push(str);
    }
    return arr.join("&");
}

2.封装Ajax

function ajax(obj){
    //指定提交方式的默认值
    obj.type = obj.type || "get";
    //设置是否异步，默认为true(异步)
    obj.async = obj.async || true;
    //设置数据的默认值
    obj.data = obj.data || null;
    if (window.XMLHttpRequest){
        //非ie
        var ajax = new XMLHttpRequest();
    }else{
        //ie
        var ajax = new ActiveXObject("Microsoft.XMLHTTP");
    }
    //区分get和post
    if (obj.type == "post"){
        ajax.open(obj.type,obj.url,obj.async);
        ajax.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
        var data = toData(obj.data);
        ajax.send(data);
    }else{
        //get  xxx?aa=xx&bb=xx
        var url = obj.url+"?"+toData(obj.data);
        ajax.open(obj.type,url,obj.async);
        ajax.send();
    }

    ajax.onreadystatechange = function (){
        if (ajax.readyState == 4){
                if (ajax.status>=200&&ajax.status<300 || ajax.status==304){
                    if (obj.success){
                        obj.success(ajax.responseText);
                    }
                }else{
                    if (obj.error){
                        obj.error(ajax.status);
                    }
                }
            }
      }    
}