Error-handling middleware in Express

Introduction

Error-handling middleware helps catch and respond to errors in your app. It keeps your app from crashing and shows friendly messages.

When you want to catch errors from routes or other middleware.

When you want to send a custom error message to users.

When you want to log errors for debugging.

When you want to handle 404 or other HTTP errors gracefully.

Syntax

Express

app.use(function (err, req, res, next) {
  // handle the error here
});

Error-handling middleware always has four parameters: err, req, res, and next.

It must be added after all other app.use() and routes.

Examples

Sends a simple 500 error message when an error happens.

Express

app.use(function (err, req, res, next) {
  res.status(500).send('Something broke!');
});

Logs the error stack to the console and sends a JSON error message.

Express

app.use(function (err, req, res, next) {
  console.error(err.stack);
  res.status(500).json({ error: err.message });
});

Sample Program

This app throws an error when you visit the home page. The error-handling middleware catches it, logs it, and sends a custom message to the browser.

Express

import express from 'express';
const app = express();

app.get('/', (req, res) => {
  throw new Error('Oops!');
});

// Error-handling middleware
app.use((err, req, res, next) => {
  console.error('Error caught:', err.message);
  res.status(500).send('Custom error: ' + err.message);
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Always place error-handling middleware last, after all routes.

Use next(err) in routes or middleware to pass errors to this handler.

Sending detailed error info to users is okay in development but avoid it in production for security.

Summary

Error-handling middleware catches errors and prevents crashes.

It has four parameters: err, req, res, next.

Place it after all other middleware and routes.

Practice

1. What is the main purpose of error-handling middleware in Express?

easy

A. To serve static files like images and CSS

B. To parse incoming JSON request bodies

C. To catch errors and prevent the server from crashing

D. To log every request made to the server

Solution

1. Step 1: Understand middleware roles

   Middleware can serve many roles, but error-handling middleware specifically catches errors.

2. Step 2: Identify error-handling middleware purpose

   Error-handling middleware prevents the server from crashing by catching errors and responding properly.

3. Final Answer:

   To catch errors and prevent the server from crashing -> Option C

4. Quick Check:

   Error-handling middleware = catch errors [OK]

Hint: Error middleware catches errors to keep server running [OK]

Common Mistakes:

• Confusing error middleware with static file serving
• Thinking it parses request bodies
• Assuming it logs requests

2. Which of the following is the correct signature for an Express error-handling middleware function?

easy

A. (err, req, res, next)

B. (req, res, next)

C. (req, res)

D. (err, res, next)

Solution

1. Step 1: Recall error middleware parameters

   Error-handling middleware always has four parameters: err, req, res, next.

2. Step 2: Match the correct parameter order

   Only (err, req, res, next) has all four parameters in the correct order.

3. Final Answer:

   (err, req, res, next) -> Option A

4. Quick Check:

   Error middleware signature = (err, req, res, next) [OK]

Hint: Error middleware always has 4 parameters starting with err [OK]

Common Mistakes:

• Using only three parameters
• Omitting the err parameter
• Wrong parameter order

3. Given the code below, what will be the response if an error occurs in the route?

const express = require('express');
const app = express();

app.get('/', (req, res) => {
  throw new Error('Oops!');
});

app.use((err, req, res, next) => {
  res.status(500).send('Error caught: ' + err.message);
});

app.listen(3000);

medium

A. The client receives 'Error caught: Oops!' with status 500

B. The server crashes and stops running

C. The client receives a 404 Not Found error

D. The client receives an empty response with status 200

Solution

1. Step 1: Identify error thrown in route

   The route throws an error with message 'Oops!'.

2. Step 2: Check error-handling middleware response

   The error middleware catches the error and sends a 500 status with message including 'Oops!'.

3. Final Answer:

   The client receives 'Error caught: Oops!' with status 500 -> Option A

4. Quick Check:

   Error thrown = caught and sent with 500 [OK]

Hint: Thrown errors go to error middleware response [OK]

Common Mistakes:

• Expecting server crash on error
• Confusing 404 with error response
• Missing error message in response

4. What is wrong with the following error-handling middleware?

app.use((req, res, next, err) => {
  res.status(500).send('Error: ' + err.message);
});

medium

A. It should be placed before routes

B. The parameters are in the wrong order

C. It is missing the next() call

D. It should not have four parameters

Solution

1. Step 1: Check parameter order for error middleware

   Error middleware must have parameters in order: err, req, res, next.

2. Step 2: Identify incorrect order in code

   The code has (req, res, next, err) which is incorrect.

3. Final Answer:

   The parameters are in the wrong order -> Option B

4. Quick Check:

   Error middleware params order = err, req, res, next [OK]

Hint: Error middleware params start with err, not req [OK]

Common Mistakes:

• Swapping err and req parameters
• Placing error middleware before routes
• Forgetting to call next() when needed

5. You want to create an error-handling middleware that logs the error message and then passes the error to the next middleware. Which of the following implementations is correct?

A) app.use((err, req, res, next) => {
  console.log(err.message);
  next(err);
});

B) app.use((req, res, next, err) => {
  console.log(err.message);
  next();
});

C) app.use((err, req, res) => {
  console.log(err.message);
  next(err);
});

D) app.use((err, req, res, next) => {
  console.log(err.message);
  res.send('Error logged');
});

hard

A. app.use((err, req, res, next) => {
console.log(err.message);
res.send('Error logged');
});

B. app.use((req, res, next, err) => {
console.log(err.message);
next();
});

C. app.use((err, req, res) => {
console.log(err.message);
next(err);
});

D. app.use((err, req, res, next) => {
console.log(err.message);
next(err);
});

Solution

1. Step 1: Recall requirements for error-handling middleware that chains

   It must have four parameters starting with err, log err.message, and call next(err) to pass the error to the next handler.

2. Step 2: Match against provided implementations

   Implementations with incorrect parameter order or count fail to receive or handle err properly; those that respond instead of calling next(err) break the chain.

3. Final Answer:

   app.use((err, req, res, next) => {
     console.log(err.message);
     next(err);
   }); -> Option D

4. Quick Check:

   4 params + next(err) = correct chaining [OK]

Hint: Error middleware must have 4 params and call next(err) to continue [OK]

Common Mistakes:

• Wrong parameter order
• Forgetting next parameter
• Not calling next(err) to pass error along
• Sending response too early