Modify response

const handler: ExportedHandler = {
  async fetch(request: Request) {
    /**
     * @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";

    /**
     * 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;
  },
};

export default handler;