Debugging Express

To see all the internal logs used in Express, set the DEBUG environment variable to express:*,router,router:* when launching your app. Routing is handled by the separate router package, so its logs live under the router namespace and are not included in express:*.

$ DEBUG=express:*,router,router:* node index.js

On Windows, use the corresponding command.

> $env:DEBUG = "express:*,router,router:*"; node index.js

Running this command on a small app with a JSON body parser and a mounted router prints the following output:

const express = require('express');

const app = express();

app.use(express.json());

const users = express.Router();
users.get('/', (req, res) => {
  res.json([]);
});

app.use('/users', users);

app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.listen(3000);

import express from 'express';

const app = express();

app.use(express.json());

const users = express.Router();
users.get('/', (req, res) => {
  res.json([]);
});

app.use('/users', users);

app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.listen(3000);

$ DEBUG=express:*,router,router:* node index.js
  express:application set "x-powered-by" to true +0ms
  express:application set "etag" to 'weak' +3ms
  express:application set "etag fn" to [Function: generateETag] +0ms
  express:application set "env" to 'development' +1ms
  express:application set "query parser" to 'simple' +0ms
  express:application set "query parser fn" to [Function: parse] +0ms
  express:application set "subdomain offset" to 2 +1ms
  express:application set "trust proxy" to false +0ms
  express:application set "trust proxy fn" to [Function: trustNone] +1ms
  express:application booting in development mode +0ms
  express:application set "view" to [Function: View] +0ms
  express:application set "views" to '/projects/example/views' +1ms
  express:application set "jsonp callback name" to 'callback' +0ms
  router use '/' jsonParser +0ms
  router:layer new '/' +0ms
  router:route new '/' +0ms
  router:layer new '/' +3ms
  router:route get / +0ms
  router:layer new '/' +1ms
  router use '/users' router +4ms
  router:layer new '/users' +0ms
  router:route new '/' +1ms
  router:layer new '/' +0ms
  router:route get / +1ms
  router:layer new '/' +1ms

When a request is then made to the app, you will see the logs specified in the Express code:

  router dispatching GET /users +712ms
  router jsonParser  : /users +1ms
  router trim prefix (/users) from url /users +2ms
  router router /users : /users +0ms
  router dispatching GET / +0ms

To see the logs only from the router implementation, set the value of DEBUG to router,router:*. Likewise, to see logs only from the application implementation, set the value of DEBUG to express:application, and so on.

Using debug in your own code

The same debug module that Express uses internally is available for your application code. Install it, create one or more loggers scoped to a namespace of your choosing, and call them wherever you would otherwise use console.log:

npm install debug

yarn add debug

pnpm add debug

bun add debug

const express = require('express');
const debug = require('debug')('myapp:server');

const app = express();

app.get('/', (req, res) => {
  debug('handling request from %s', req.ip);
  res.send('Hello World!');
});

app.listen(3000, () => {
  debug('listening on port 3000');
});

import express from 'express';
import debugModule from 'debug';

const debug = debugModule('myapp:server');
const app = express();

app.get('/', (req, res) => {
  debug('handling request from %s', req.ip);
  res.send('Hello World!');
});

app.listen(3000, () => {
  debug('listening on port 3000');
});

These statements print nothing by default. Enable them through the same DEBUG environment variable, using your own namespace:

$ DEBUG=myapp:* node index.js

You can specify more than one debug namespace by assigning a comma-separated list of names:

$ DEBUG=myapp:*,router node index.js

Setting DEBUG in your IDE

The DEBUG environment variable can be set by any process launcher that supports environment variables, including the run and debug configurations of your IDE. For example, in VS Code you can set it in the env property of a launch configuration:

{
  "type": "node",
  "request": "launch",
  "name": "Launch Express app",
  "program": "${workspaceFolder}/index.js",
  "env": { "DEBUG": "express:*,router,router:*" }
}

See Node.js debugging in VS Code for the full list of launch options.

Using the Node.js inspector

Debug logs show what the app did; for stepping through your code with breakpoints, use the built-in Node.js inspector. Start your app with the --inspect flag:

$ node --inspect index.js

Then attach a debugging client, such as Chrome DevTools (open chrome://inspect), VS Code, or any other inspector-capable tool, to set breakpoints in your route handlers and middleware, step through code, and inspect variables.

If you need to debug something that happens during startup, use --inspect-brk instead, which pauses execution on the first line until a debugger attaches. See the Node.js debugging guide for details.

Debugging the HTTP layer

Express runs on top of the Node.js http module, which has its own debugging facilities that work with any Express app.

Setting the NODE_DEBUG environment variable to http makes Node.js print internal logs from the HTTP layer, such as connection handling and socket events:

$ NODE_DEBUG=http node index.js

You can combine it with other subsystems, such as net or stream, in a comma-separated list. Be aware that this output can expose sensitive data such as authentication headers, so use it only in development.

Diagnostics channels

Node.js publishes an event for every HTTP request through the diagnostics_channel module, which you can subscribe to without patching Express or adding middleware:

const { subscribe } = require('node:diagnostics_channel');

subscribe('http.server.request.start', ({ request }) => {
  console.log(`${request.method} ${request.url}`);
});

import { subscribe } from 'node:diagnostics_channel';

subscribe('http.server.request.start', ({ request }) => {
  console.log(`${request.method} ${request.url}`);
});

Other built-in channels cover the rest of the request lifecycle, such as http.server.response.finish, and the client side of http.request calls. See the diagnostics_channel documentation for the full list.

Advanced options

When running through Node.js, you can set a few environment variables that will change the behavior of the debug logging: