//调用方式:

 /*

   ajaxPrmomise({

   url:,

   method:,

   headers:{}

 }).then(res=>{})

 */

 ;(function(window){

   //设置默认的参数配置项

   let _default = {

     url:'',

     baseURL:'',

     method:'GET',

     params:null, //get请求基于问号传参方式传递给服务器的内容

     headers:{},

     timeout:500,

     dataType:'json',

     data:null,  //post请求基于请求主体传参方式传递给服务器的内容

     cache:true   //是否缓存数据

   };

   //resolve,reject,response,fn

   let _setResponseMsg = function _setResponseMsg(...rest){

       let [

         resolve,

         reject,

         xhr,

         fn

       ] = rest,

       res = null,

       response = xhr;

       res.data = fn(response.responseText);

       res.status = response.status;

       res.statusText = response.statusText;

       res.xhr = response;

       res.headers = response.getAllResponseHeaders();

       resolve(res)

   }

   let _convertJSON = function _convertJSON(result){

       let data = "";

       try {

         data = JSON.parse(result);

       } catch (e) {

         reject(e)

       }

       return data

   }

   let _convertString = function _convertString(result){

       return result;

   }

   let _array_to_str = function _array_to_str(key,arr){

       let params = "";

       for(let i0;i<arr.length;i++){

         params += `${key}=${arr[i]}&`;

       }

       return params.slice(0,-1)

   }

   let _convertParams = function _convertParams(cache,params){

     let str = "?",

         time_str = cache ? "":(+(new Date())+"");

       if(typeof params == 'object' && Object.keys(params).length > 0){

           for(let key in params){

             if(typeof params[key] == 'object' && (length in params[key])){

               str += _array_to_str(key,params[key])

             }

             str += `${key}=${params[key]}`;

           }

       }

       return str+"&_="+time_str;

   }

   let _setHeader = function _setHeader(...res){

     let [xhr,headers] = res;

     for(let key in headers){

       if(headers.hasOwnProperty(key))

         xhr.setRequestHeader(key,headers[key])

     }

   }

   //基于promise管理ajax请求

   let ajaxPrmomise = function ajaxPrmomise(options={}){

       // 这里传递的options包含默认配置信息和用户基于暴露的_default修改后的信息

       for(let key in _default){

         if(key in options && _default.hasOwnProperty(key)){

             options[key] = _default[key]

         }

       }

       //=>这里可以写成

       options = {..._default,...options};

       let {

         url,

         method,

         baseURL,

         data,

         dataType,

         headers,

         cache,

         params

       } = options;

       let xhr,send_data;

       return new Promise((resolve,reject)=>{

           xhr = new XMLHttpRequest();

           _setHeader(xhr,headers);

           send_data = method.toLowerCase() == 'get' ? _convertParams(cache,params):(method.toLowerCase() == 'post'? data:"");

           all_url = baseURL + url + send_data;

           xhr.open(method,all_url);

           xhr.onreadystatechange = function(){

             if(xhr.readyState == 4){

               if(/^[23]\d{2}$/.test(xhr.status)){

                   dataType = dataType.toUpperCase();

                   if(dataType == "json")

                     _setResponseMsg(resolve,reject,xhr,_convertJSON)

                   _setResponseMsg(resolve,reject,xhr,_convertString)

                 }

               }

             }

             ajaxPrmomise.xhr = xhr;

             xhr.send(send_data);

           })

       }

   //修改默认配置，可以通过自己设置一些配置来覆盖默认配置

   ajaxPrmomise.defaults = _default;

 ["get","head","options",""].forEach((method,indexs)=>{

   ajaxPrmomise[method] = function(url,options){

       return ajaxPrmomise({

         ...options,

         url:url,

         method:method

       })

     }

 });

 ["post","put"].forEach((method,index)=>{

   ajaxPrmomise[method] = function(url,data,options){

     return ajaxPrmomise({

       ...options,

       url:url,

       data:data,

       method:method

     })

   }

 })

   window.ajaxPrmomise = ajaxPrmomise;

 }(window))