// 浏览器使用图片格式
const broswerFormat = {
  Chrome: 'webp',
  Opera: 'webp',
  Firefox: 'webp',
  Safari: 'jp2',
  Edge: 'webp',
  IE: 'jxr'
};

addEventListener('fetch', event => {
  // 当函数代码抛出未处理的异常时，边缘函数会将此请求转发回源站 
  event.passThroughOnException();
  event.respondWith(handleEvent(event));
});

async function handleEvent(event) {
  const { request } = event;
  const userAgent = request.headers.get('user-agent');
  const bs = getBroswer(userAgent);
  const format = broswerFormat[bs];
  
  // 无需转换图片格式
  if (!format) {
    return fetch(request);
  }

  // 图片格式转换
  const response = await fetch(request, {
    eo: {
      image: {
        format
      }
    }
  });

  // 设置响应头
  response.headers.set('x-ef-format', format);
  
  return response;
}

function getBroswer(userAgent) {
  if (/Edg/i.test(userAgent)) {
    return 'Edge'
  }
  if (/Trident/i.test(userAgent)) {
    return 'IE'
  }
  if (/Firefox/i.test(userAgent)) {
    return 'Firefox';
  }
  if (/Chrome/i.test(userAgent)) {
    return 'Chrome';
  }
  if (/Opera|OPR/i.test(userAgent)) {
    return 'Opera';
  }
  if (/Safari/i.test(userAgent)) {
    return 'Safari'
  }
}