获取Multipart/related类型图像/jpeg到Base64 JavaScript

function getMultipartRelatedImage(url) {
  var xhr = new XMLHttpRequest();
  xhr.open('GET', url, true);
  xhr.responseType = 'arraybuffer';

  xhr.onload = function() {
    if (xhr.status === 200) {
      var contentType = xhr.getResponseHeader('Content-Type');
      if (contentType.indexOf('multipart/related') !== -1) {
        var boundary = contentType.match(/boundary=(?:"([^"]+)"|([^;]+))/i);
        if (boundary) {
          boundary = boundary[1] || boundary[2];
          var parts = parseMultipartRelated(xhr.response, boundary);
          var imagePart = findImagePart(parts);
          if (imagePart) {
            var base64Image = arrayBufferToBase64(imagePart.data);
            // 使用base64Image进行后续操作
          }
        }
      }
    }
  };

  xhr.send();
}

function parseMultipartRelated(data, boundary) {
  var parts = [];
  var boundaryRegExp = new RegExp('--' + boundary + '(?:--)?\\r?\\n', 'g');
  var matches;
  while ((matches = boundaryRegExp.exec(data)) !== null) {
    var start = matches.index + matches[0].length;
    var end = boundaryRegExp.lastIndex - matches[0].length - 2;
    var partData = data.slice(start, end);
    parts.push(parsePart(partData));
  }
  return parts;
}

function parsePart(data) {
  var headersEnd = data.indexOf('\r\n\r\n');
  var headers = data.slice(0, headersEnd);
  var body = data.slice(headersEnd + 4);
  var part = {
    headers: parseHeaders(headers),
    data: body
  };
  return part;
}

function parseHeaders(headers) {
  var lines = headers.split('\r\n');
  var headersObj = {};
  for (var i = 0; i < lines.length; i++) {
    var line = lines[i];
    var separatorIndex = line.indexOf(':');
    var key = line.slice(0, separatorIndex).trim();
    var value = line.slice(separatorIndex + 1).trim();
    headersObj[key] = value;
  }
  return headersObj;
}

function findImagePart(parts) {
  for (var i = 0; i < parts.length; i++) {
    var part = parts[i];
    if (part.headers['Content-Type'] && part.headers['Content-Type'].indexOf('image/jpeg') !== -1) {
      return part;
    }
  }
  return null;
}

function arrayBufferToBase64(buffer) {
  var binary = '';
  var bytes = new Uint8Array(buffer);
  for (var i = 0; i < bytes.length; i++) {
    binary += String.fromCharCode(bytes[i]);
  }
  return btoa(binary);
}