原生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.readyState === 4 && xhr.status === 200){
                    var json = JSON.parse(xhr.responseText)
                    resolve(json)
                }
                else if(xhr.readyState === 4 && xhr.status !== 200){
                    reject('error')
                }
            }
        })
    }