使用Promise来封装一个Ajax

代补充

function PromiseAjax(_methods,_url,_data){
    return new Promise((resolve,reject) => {
        let xhr = new XMLHttpRequest()
        data = _data || null
        methods = _methods ||'get'

        xhr.open(_methods,_url,true)
        xhr.onreadystatechange = function(){
            if(xhr.readyState == 4){
                if(xhr.status == 200){
                    resolve(xhr.responseText)
                }else{
                    reject(xhr.responseText)
                }
            }
        }
        xhr.send(_data)

    })
}