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

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

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
});

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

Hint

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

Practice

1. In Express, how do you handle synchronous errors inside a route handler?

easy

A. Use setTimeout to catch errors asynchronously

B. Use async/await without try...catch

C. Ignore errors and let the server crash

D. Use a try...catch block and call next(error) inside catch

Solution

1. Step 1: Understand synchronous error handling

   Synchronous errors happen immediately and can be caught with try...catch blocks.

2. Step 2: Pass error to Express error handler

   Inside catch, call next(error) to forward the error to Express's error middleware.

3. Final Answer:

   Use a try...catch block and call next(error) inside catch -> Option D

4. Quick Check:

   try...catch + next(error) = A [OK]

Hint: Catch sync errors with try...catch and call next(error) [OK]

Common Mistakes:

• Not using try...catch for synchronous errors
• Calling next() without an error object
• Using async/await without try...catch for sync errors

2. Which of the following is the correct syntax to pass a synchronous error to the next middleware in Express?

easy

A. next(new Error('Something went wrong'))

B. throw new Error('Something went wrong')

C. res.send(new Error('Something went wrong'))

D. console.error('Something went wrong')

Solution

1. Step 1: Identify how to forward errors in Express

   Express expects errors to be passed to next() as an argument to trigger error middleware.

2. Step 2: Check syntax correctness

   Calling next(new Error('...')) correctly passes the error object to Express.

3. Final Answer:

   next(new Error('Something went wrong')) -> Option A

4. Quick Check:

   next(error) syntax = B [OK]

Hint: Use next(error) to forward errors in Express [OK]

Common Mistakes:

• Throwing error without try...catch in sync code
• Sending error object in response instead of next()
• Logging error without forwarding it

3. What will be the output when this Express route is called?

app.get('/test', (req, res, next) => {
  try {
    throw new Error('Fail');
  } catch (err) {
    next(err);
  }
});

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

medium

A. The server crashes with uncaught exception

B. The client receives a 200 status with empty body

C. The client receives a 500 status with message 'Fail'

D. The client receives a 404 Not Found error

Solution

1. Step 1: Analyze error throwing and catching

   The route throws an error inside try, caught immediately, then passed to next(err).

2. Step 2: Understand error middleware response

   The error middleware sends a 500 status with the error message 'Fail' to the client.

3. Final Answer:

   The client receives a 500 status with message 'Fail' -> Option C

4. Quick Check:

   next(err) triggers error middleware = A [OK]

Hint: Errors passed to next() trigger error middleware response [OK]

Common Mistakes:

• Assuming error crashes server without middleware
• Expecting 200 status despite error
• Confusing error middleware with normal middleware

4. Identify the error in this Express route for synchronous error handling:

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

medium

A. No error, code works fine

B. Error is thrown but not caught or passed to next()

C. Error middleware is missing

D. next() is called after throw, which is unreachable

Solution

1. Step 1: Check error throwing without try...catch

   The error is thrown synchronously but not caught, so Express cannot handle it properly.

2. Step 2: Understand consequence of uncaught error

   Without try...catch and calling next(err), the server may crash or not handle error gracefully.

3. Final Answer:

   Error is thrown but not caught or passed to next() -> Option B

4. Quick Check:

   Throw without catch + next(err) = D [OK]

Hint: Always catch sync errors and call next(error) [OK]

Common Mistakes:

• Throwing error without try...catch
• Calling next() after throw (unreachable code)
• Assuming Express auto-catches thrown errors

5. You want to handle synchronous errors in multiple routes without repeating try...catch blocks. Which Express feature helps you achieve this cleanly?

hard

A. Create a wrapper function that catches errors and calls next(err)

B. Use setTimeout to delay error throwing

C. Call res.end() immediately to avoid errors

D. Use global variables to store errors

Solution

1. Step 1: Recognize repetitive try...catch problem

   Wrapping every route in try...catch is repetitive and cluttered.

2. Step 2: Use a wrapper function to centralize error catching

   A wrapper function can run the route handler and catch sync errors, forwarding them with next(err).

3. Final Answer:

   Create a wrapper function that catches errors and calls next(err) -> Option A

4. Quick Check:

   Wrapper function for sync error handling = C [OK]

Hint: Use wrapper functions to catch and forward sync errors [OK]

Common Mistakes:

• Trying to delay errors with setTimeout
• Ignoring errors by ending response early
• Using global variables for error handling