Error-handling middleware in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does error-handling middleware affect request flow in Express?

Consider an Express app with normal middleware and error-handling middleware. What happens when an error is passed to next()?

Node.js

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

app.use((req, res, next) => {
  if (req.query.fail) {
    next(new Error('Failure triggered'));
  } else {
    next();
  }
});

app.use((req, res, next) => {
  res.send('Success');
});

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

A The error-handling middleware runs only if <code>res.send()</code> is not called first.

B The error is ignored and the normal middleware continues to run, sending 'Success'.

C The app crashes immediately when <code>next()</code> is called with an error.

D If <code>next()</code> is called with an error, the error-handling middleware runs and sends a 500 response.

💡 Hint

Think about how Express decides which middleware to run when next() receives an error.

When next() is called with an error, Express skips normal middleware and runs error-handling middleware. This middleware has four arguments (err, req, res, next) and handles the error, often sending a 500 response.

📝 Syntax

intermediate

Identify the syntax error in this error-handling middleware

Which option contains a syntax error in defining error-handling middleware in Express?

Node.js

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

A Missing <code>next</code> parameter in the middleware function signature.

B Using <code>function</code> keyword instead of arrow function.

C Middleware function has four parameters but <code>err</code> is not first.

D Calling <code>res.status()</code> without <code>send()</code>.

💡 Hint

Error-handling middleware must have exactly four parameters.

Error-handling middleware in Express must have four parameters: err, req, res, next. Missing the next parameter causes Express to treat it as normal middleware, so it won't catch errors properly.

🔧 Debug

advanced

Why does this error-handling middleware never run?

Given this Express app, why does the error-handling middleware never respond?

Node.js

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

app.use((req, res, next) => {
  next(new Error('Oops'));
});

app.use((req, res, next) => {
  res.send('Hello');
});

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

A The error-handling middleware is missing the <code>next</code> parameter.

B The error-handling middleware has parameters in the wrong order; <code>err</code> must be first.

C The error-handling middleware is placed before the normal middleware.

D The error-handling middleware uses arrow function syntax which is not supported.

💡 Hint

Check the order of parameters in error-handling middleware.

Error-handling middleware must have four parameters in this order: err, req, res, next. The code has req, res, next, err, so Express does not recognize it as error-handling middleware and never calls it.

❓ state_output

advanced

What is the response status and body when an error is thrown inside async middleware?

Consider this Express async middleware that throws an error. What response does the client receive?

Node.js

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

app.use(async (req, res, next) => {
  throw new Error('Async failure');
});

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

A The app crashes with an unhandled promise rejection error.

B The client receives a 500 status with body 'Caught: Async failure'.

C The client receives no response and the request times out.

D The client receives a 200 status with empty body.

💡 Hint

Think about how Express handles errors thrown inside async functions.

Express does not automatically catch errors thrown inside async middleware unless next(err) is called or the async function is wrapped. Throwing an error inside async middleware causes an unhandled promise rejection and crashes the app.

🧠 Conceptual

expert

Which statement about error-handling middleware in Express is TRUE?

Choose the correct statement about error-handling middleware behavior in Express.

A Calling <code>next()</code> without arguments inside error-handling middleware passes control to the next error handler.

B Error-handling middleware must be the last middleware added to the app to catch all errors.

C Error-handling middleware is identified by having exactly four parameters: <code>err, req, res, next</code>.

D If an error occurs in normal middleware, Express automatically skips all remaining middleware and sends a 500 response.

💡 Hint

Recall how Express detects error-handling middleware.

Express identifies error-handling middleware by the function signature having four parameters: err, req, res, next. This is how it knows to call it when an error occurs. Other statements are false: error handlers do not have to be last, next() without arguments inside error handlers passes control to normal middleware, and Express does not automatically send 500 responses without error handlers.