Synchronous error handling in Express - Mini Project: Build & Apply

Synchronous error handling in Express

📖 Scenario: You are building a simple Express server that handles user requests. Sometimes, errors happen during request processing. You want to catch these errors synchronously and send a proper error response.

🎯 Goal: Create an Express server with a route that throws a synchronous error. Add error handling middleware to catch the error and respond with a JSON error message.

📋 What You'll Learn

Create an Express app instance called app

Add a GET route at /error that throws a synchronous error with message 'Something went wrong!'

Add an error handling middleware function with four parameters: err, req, res, next

In the error handler, respond with status code 500 and JSON { error: err.message }

💡 Why This Matters

🌍 Real World

Web servers often encounter errors during request processing. Handling errors properly ensures users get clear messages and the server stays stable.

💼 Career

Knowing how to handle errors in Express is essential for backend developers building reliable Node.js web applications.

1

Set up Express app and route

Create an Express app instance called app. Add a GET route at /error that throws a new Error with the message 'Something went wrong!'.

Express

import express from 'express';

// Create Express app instance
// Add GET route at /error that throws an error
// Your code here

Need a hint?

Use const app = express() to create the app. Use app.get('/error', (req, res) => { throw new Error('Something went wrong!') }) to add the route.

2

Add error handling middleware

Add an error handling middleware function with parameters err, req, res, next to the app. For now, just add the function without implementation.

Express

import express from 'express';

const app = express();

app.get('/error', (req, res) => {
  throw new Error('Something went wrong!');
});

// Add error handling middleware function with (err, req, res, next)
// Your code here

Need a hint?

Use app.use to add the error handler middleware with four parameters.

3

Implement error handler response

Inside the error handling middleware, respond with status code 500 and JSON object { error: err.message } using res.status(500).json({ error: err.message }).

Express

import express from 'express';

const app = express();

app.get('/error', (req, res) => {
  throw new Error('Something went wrong!');
});

app.use((err, req, res, next) => {
  // Respond with status 500 and JSON error message
  // Your code here
});

Need a hint?

Use res.status(500).json({ error: err.message }) to send the error response.

4

Start the Express server

Add code to start the Express server by calling app.listen(3000) to listen on port 3000.

Express

import express from 'express';

const app = express();

app.get('/error', (req, res) => {
  throw new Error('Something went wrong!');
});

app.use((err, req, res, next) => {
  res.status(500).json({ error: err.message });
});

// Start the server on port 3000
// Your code here

Need a hint?

Use app.listen(3000) to start the server on port 3000.