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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);
`Response header "${headerNameDst}" was set to "${response.headers.get(headerNameDst)}"`
return response;
addEventListener('fetch', event => {