promise封装的ajax

var  myNewAjax=function(url){

  return new Promise(function(resolve,reject){

      var xhr = new XMLHttpRequest();

      xhr.open('get',url);

      xhr.send(data);

      xhr.onreadystatechange=function(){

           if(xhr.status==200&&readyState==4){

                var json=JSON.parse(xhr.responseText);

                resolve(json)

           }else if(xhr.readyState==4&&xhr.status!=200){

                reject('error');

           }

      }

  })

}