Promise封装一个ajax

                                                       let ajax=(obj)=>{
                                                       return new Promise((resolve,reject)=>{
                                                       let method=obj.method||'GET';
                                                       let xhr=null;
                                                       if(window.XMLHttpRequest){
                                                       xhr=new XMLHttpRequest();
                                                       }else{
                                                       xhr=new ActiveObject('Microsoft.XMLHTTP);
                                                       }
                                                       xhr.onreadystatechange=()=>{
                                                       if(xhr.readyState==4&&xhr.status==200){
                                                          reslove(xhr.responseText);
                                                          }else{
                                                          reject(xhr.status);
                                                          }
                                                          }
                                                          if(method=='POST'){
                                                          xhr.open('POST',obj.url,true);
                                                          }
                                                          else{
                                                                          let query = '';
                                                               for (let key in obj.data) {
                                                               query += '&' + encodeURIComponent(key) + "=" + encodeURIComponent(obj.data[key]);
                                                    }
                                                             query.substring(1);
                                          xhr.open('GET', obj.url + '?' + query, true);
                                             xhr.send();
                                                          }
                                                          xhr.timeout=1000;//设置超时时间
                                                          });
                                                          }