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');

}

}

})

}