Modify response

/**
 * @param {string} headerNameSrc Header to get the new value from
 * @param {string} headerNameDst Header to set based off of value in src
 */
const headerNameSrc = 'foo'; //"Orig-Header"
const headerNameDst = 'Last-Modified';

async function handleRequest(request) {
  /**
   * Response properties are immutable. To change them, construct a new
   * Response and pass modified status or statusText in the ResponseInit
   * object. Response headers can be modified through the headers `set` method.
   */
  const originalResponse = await fetch(request);

  // Change status and statusText, but preserve body and headers
  let response = new Response(originalResponse.body, {
    status: 500,
    statusText: 'some message',
    headers: originalResponse.headers,
  });

  // Change response body by adding the foo prop
  const originalBody = await originalResponse.json();
  const body = JSON.stringify({ foo: 'bar', ...originalBody });
  response = new Response(body, response);

  // Add a header using set method
  response.headers.set('foo', 'bar');

  // Set destination header to the value of the source header
  const src = response.headers.get(headerNameSrc);

  if (src != null) {
    response.headers.set(headerNameDst, src);
    console.log(
      `Response header "${headerNameDst}" was set to "${response.headers.get(headerNameDst)}"`
    );
  }
  return response;
}

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