---
title: Modify response
description: Fetch and modify response properties which are immutable by creating a copy first.
image: https://edgetunnel-b2h.pages.dev/dev-products-preview.png
---

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# Modify response

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

If you want to get started quickly, click on the button below.

[![Deploy to Cloudflare](https://deploy.workers.cloudflare.com/button)](https://deploy.workers.cloudflare.com/?url=https://github.com/cloudflare/docs-examples/tree/main/workers/modify-response)

This creates a repository in your GitHub account and deploys the application to Cloudflare Workers.

* [  JavaScript ](#tab-panel-12564)
* [  TypeScript ](#tab-panel-12565)
* [  Python ](#tab-panel-12566)
* [  Hono ](#tab-panel-12567)

**JavaScript**

```js
export default {
  async fetch(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;
  },
};
```

**TypeScript**

```ts
export default {
  async fetch(request): Promise<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";


    /**
     * 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;
  },
} satisfies ExportedHandler;
```

**Python**

```py
from workers import WorkerEntrypoint, Response, fetch
import json


class Default(WorkerEntrypoint):
    async def fetch(self, request):
        header_name_src = "foo" # Header to get the new value from
        header_name_dst = "Last-Modified" # Header to set based off of value in src


        # Response properties are immutable. To change them, construct a new response
        original_response = await fetch(request)


        # Change status and statusText, but preserve body and headers
        response = Response(original_response.body, status=500, status_text="some message", headers=original_response.headers)


        # Change response body by adding the foo prop
        new_body = await original_response.json()
        new_body["foo"] = "bar"
        response.replace_body(json.dumps(new_body))


        # Add a new header
        response.headers["foo"] = "bar"


        # Set destination header to the value of the source header
        src = response.headers[header_name_src]


        if src is not None:
            response.headers[header_name_dst] = src
            print(f'Response header {header_name_dst} was set to {response.headers[header_name_dst]}')


        return response
```

**TypeScript**

```ts
import { Hono } from 'hono';


const app = new Hono();


app.get('*', async (c) => {
  /**
   * Header configuration
   */
  const headerNameSrc = "foo"; // Header to get the new value from
  const headerNameDst = "Last-Modified"; // Header to set based off of value in src


  /**
   * Response properties are immutable. With Hono, we can modify the response
   * by creating custom response objects.
   */
  const originalResponse = await fetch(c.req.raw);


  // Get the JSON body from the original response
  const originalBody = await originalResponse.json();


  // Modify the body by adding a new property
  const modifiedBody = {
    foo: "bar",
    ...originalBody
  };


  // Create a new custom response with modified status, headers, and body
  const response = new Response(JSON.stringify(modifiedBody), {
    status: 500,
    statusText: "some message",
    headers: originalResponse.headers,
  });


  // 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 app;
```

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