将原生的ajax封装成promise

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 && xhr.readyState === 4){
                resolve(json);
            }
            else if(xhr.readyState === 4 && xhr.status !== 200){
                reject('error');
            }
        }
    })
}