如何从XMLHttpRequest获得进展

var xhr = new XMLHttpRequest();
xhr.upload.addEventListener("progress", function(event) {
  if (event.lengthComputable) {
    var percentComplete = Math.round(event.loaded * 100 / event.total);
    console.log("上传进度：" + percentComplete + "%");
  }
});
xhr.onreadystatechange = function() {
  if (xhr.readyState === 4) {
    if (xhr.status === 200) {
      console.log("上传成功");
    } else {
      console.log("上传失败");
    }
  }
};
xhr.open("POST", "your_server_url", true);
xhr.send(formData);