--- title: Debugging logs description: Send debugging information in an errored response to a logging service. image: https://developers.cloudflare.com/dev-products-preview.png --- > Documentation Index > Fetch the complete documentation index at: https://developers.cloudflare.com/workers/llms.txt > Use this file to discover all available pages before exploring further. [Skip to content](#%5Ftop) ### Tags [ Debugging ](https://developers.cloudflare.com/search/?tags=Debugging)[ JavaScript ](https://developers.cloudflare.com/search/?tags=JavaScript)[ TypeScript ](https://developers.cloudflare.com/search/?tags=TypeScript)[ Python ](https://developers.cloudflare.com/search/?tags=Python) # Debugging logs **Last reviewed:** over 5 years ago 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-9281) * [ TypeScript ](#tab-panel-9282) * [ Python ](#tab-panel-9283) * [ Hono ](#tab-panel-9284) JavaScript ``` 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 ``` interface Env {} export default { async fetch(request, env, ctx): Promise { // 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; ``` Python ``` 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 ``` 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; ``` ```json {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"item":{"@id":"/directory/","name":"Directory"}},{"@type":"ListItem","position":2,"item":{"@id":"/workers/","name":"Workers"}},{"@type":"ListItem","position":3,"item":{"@id":"/workers/examples/","name":"Examples"}},{"@type":"ListItem","position":4,"item":{"@id":"/workers/examples/debugging-logs/","name":"Debugging logs"}}]} ```