---
title: Debugging logs
description: Send debugging information in an errored response to a logging service.
image: https://edgetunnel-b2h.pages.dev/dev-products-preview.png
---

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# Debugging logs

Send debugging information in an errored response to a logging service.

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/debugging-logs)

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

* [  JavaScript ](#tab-panel-12520)
* [  TypeScript ](#tab-panel-12521)
* [  Python ](#tab-panel-12522)
* [  Hono ](#tab-panel-12523)

**JavaScript**

```js
export default {
  async fetch(request, env, ctx) {
    // Service configured to receive logs
    const LOG_URL = "https://log-service.example.com/";


    async function postLog(data) {
      return await fetch(LOG_URL, {
        method: "POST",
        body: data,
      });
    }


    let response;


    try {
      response = await fetch(request);
      if (!response.ok && !response.redirected) {
        const body = await response.text();
        throw new Error(
          "Bad response at origin. Status: " +
            response.status +
            " Body: " +
            // Ensure the string is small enough to be a header
            body.trim().substring(0, 10),
        );
      }
    } catch (err) {
      // Without ctx.waitUntil(), your fetch() to Cloudflare's
      // logging service may or may not complete
      ctx.waitUntil(postLog(err.toString()));
      const stack = JSON.stringify(err.stack) || err;
      // Copy the response and initialize body to the stack trace
      response = new Response(stack, response);
      // Add the error stack into a header to find out what happened
      response.headers.set("X-Debug-stack", stack);
      response.headers.set("X-Debug-err", err);
    }
    return response;
  },
};
```

**TypeScript**

```ts
interface Env {}
export default {
  async fetch(request, env, ctx): Promise<Response> {
    // Service configured to receive logs
    const LOG_URL = "https://log-service.example.com/";


    async function postLog(data) {
      return await fetch(LOG_URL, {
        method: "POST",
        body: data,
      });
    }


    let response;


    try {
      response = await fetch(request);
      if (!response.ok && !response.redirected) {
        const body = await response.text();
        throw new Error(
          "Bad response at origin. Status: " +
            response.status +
            " Body: " +
            // Ensure the string is small enough to be a header
            body.trim().substring(0, 10),
        );
      }
    } catch (err) {
      // Without ctx.waitUntil(), your fetch() to Cloudflare's
      // logging service may or may not complete
      ctx.waitUntil(postLog(err.toString()));
      const stack = JSON.stringify(err.stack) || err;
      // Copy the response and initialize body to the stack trace
      response = new Response(stack, response);
      // Add the error stack into a header to find out what happened
      response.headers.set("X-Debug-stack", stack);
      response.headers.set("X-Debug-err", err);
    }
    return response;
  },
} satisfies ExportedHandler<Env>;
```

**Python**

```py
from workers import WorkerEntrypoint
from pyodide.ffi import create_proxy
from js import Response, fetch


async def post_log(data):
  log_url = "https://log-service.example.com/"
  await fetch(log_url, method="POST", body=data)


class Default(WorkerEntrypoint):
    async def fetch(self, request):
        # Service configured to receive logs
        response = await fetch(request)


        try:
            if not response.ok and not response.redirected:
                body = await response.text()
            # Simulating an error. Ensure the string is small enough to be a header
            raise Exception(f'Bad response at origin. Status:{response.status} Body:{body.strip()[:10]}')
        except Exception as e:
            # Without ctx.waitUntil(), your fetch() to Cloudflare's
            # logging service may or may not complete
            self.ctx.waitUntil(create_proxy(post_log(str(e))))
            # Copy the response and add to header
            response = Response.new(stack, response)
            response.headers["X-Debug-err"] = str(e)


        return response
```

**TypeScript**

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


// Define the environment with appropriate types
interface Env {}


const app = new Hono<{ Bindings: Env }>();


// Service configured to receive logs
const LOG_URL = "https://log-service.example.com/";


// Function to post logs to an external service
async function postLog(data: string) {
  return await fetch(LOG_URL, {
    method: "POST",
    body: data,
  });
}


// Middleware to handle error logging
app.use('*', async (c, next) => {
  try {
    // Process the request with the next handler
    await next();


    // After processing, check if the response indicates an error
    if (c.res && (!c.res.ok && !c.res.redirected)) {
      const body = await c.res.clone().text();
      throw new Error(
        "Bad response at origin. Status: " +
        c.res.status +
        " Body: " +
        // Ensure the string is small enough to be a header
        body.trim().substring(0, 10)
      );
    }


  } catch (err) {
    // Without waitUntil, the fetch to the logging service may not complete
    c.executionCtx.waitUntil(
      postLog(err.toString())
    );


    // Get the error stack or error itself
    const stack = JSON.stringify(err.stack) || err.toString();


    // Create a new response with the error information
    const response = c.res ?
      new Response(stack, {
        status: c.res.status,
        headers: c.res.headers
      }) :
      new Response(stack, { status: 500 });


    // Add debug headers
    response.headers.set("X-Debug-stack", stack);
    response.headers.set("X-Debug-err", err.toString());


    // Set the modified response
    c.res = response;
  }
});


// Default route handler that passes requests through
app.all('*', async (c) => {
  return fetch(c.req.raw);
});


export default app;
```

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