请使用Promise封装Ajax操作原始的Ajax操作如下：varonSu...

Question

请使用Promise封装Ajax操作
原始的Ajax操作如下：

var onSuccess = function(result){} //成功的回调
var onFail = function(error){} //失败的回调
var req = new XMLHttpRequest()
req.open("POST", "www.baidu.com", true)
req.onload = function(){
  if(req.readyState === 4 && req.status === 200){
    onSuccess(req.response)
  } else {
    onFail(req.statusText)
  }
}
req.onerror = function(){
  onFail(Error("网络异常"))
}