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Modify response

Fetch and modify response properties which are immutable by creating a copy first.

/** * @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))})