Modifying a Response Header

async function handleRequest(event) {
  const { request } = event;
  const urlInfo = new URL(request.url);
  
  const proxyRequest = new Request(`https://www.example.com${urlInfo.pathname}${urlInfo.search}`, {
    method: request.method,
    body: request.body,
    headers: request.headers,
    copyHeaders: true,
  });
  proxyRequest.headers.set('Host', 'www.example.com');
  
  // fetch reverse proxy
  const response = await fetch(proxyRequest);

  /** Add custom response headers **/
  // Specify which origins are allowed to access resources
  response.headers.append('Access-Control-Allow-Origin', '*');
  // Specify which HTTP methods (such as GET, POST, etc.) are allowed to access resources
  response.headers.append('Access-Control-Allow-Methods', 'GET,POST');
  // Specify which HTTP headers can appear in the request header
  response.headers.append('Access-Control-Allow-Headers', 'Authorization');
  // How long the result of the pre-flight request can be cached
  response.headers.append('Access-Control-Max-Age', '86400');
  
  /** Delete response headers **/
  response.headers.delete('X-Cache');
  
  return response;
}

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event));
});