const xhr = new XMLHttpRequest();xhr.open('GET', 'url');// 如果已指明，responseType 必须是空字符串或 "docuemnt" xhr.responseType = 'blog';xhr.onload = ()=> {  if (xhr.readyState === xhr.DONE && xhr.status === 200) {      // 转成dataUrl      // window.URL.createObjectURL(result)      // 转成base64      const reader = new FileReader();      reader.onloadend = () => resolve(reader.result);      reader.readAsDataURL(result);  }};xhr.send(null);