---
description: Fetch and modify response properties which are immutable by creating a copy first.
title: Modify response
image: https://developers.cloudflare.com/og-docs.png
---

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# Modify response

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

Last updated Apr 23, 2026|Copy as Markdown|[View as Markdown](https://developers.cloudflare.com/workers/examples/modify-response/index.md)|[Agent setup](https://developers.cloudflare.com/agent-setup/)

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```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;
	},
};
```

```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;
```

```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
```

```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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