404 Not Found handler in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a request hits this 404 handler?

Consider this Express 404 handler middleware. What will the client receive if they request a non-existent route?

Express

app.use((req, res) => {
  res.status(404).send('Page not found');
});

A The client receives a 500 status with an error message.

B The client receives a 200 status with the text 'Page not found'.

C The client receives a 404 status with an empty response body.

D The client receives a 404 status with the text 'Page not found'.

💡 Hint

Look at the status code set and the response sent.

This middleware sets the HTTP status to 404 and sends the text 'Page not found'. So the client gets a 404 status with that message.

📝 Syntax

intermediate

Which 404 handler code snippet is syntactically correct?

Identify the valid Express 404 handler middleware from the options below.

A

app.use((req, res, next) => {
  res.status(404).send('Not found');
});

B

app.use((req, res) => {
  res.status(404).send('Not found');
});

C

app.use((req, res, next) => {
  res.status(404).send('Not found')
);

D

app.use((req, res, next) => {
  res.status(404).send('Not found');
);

💡 Hint

Check for balanced parentheses and braces.

Option A has correct syntax with balanced braces and parentheses. Others have missing or misplaced characters causing syntax errors.

🔧 Debug

advanced

Why doesn't this 404 handler run for requests to '/hello'?

Given this Express app code, why does the 404 handler not get called when requesting '/hello'?

Express

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

app.use((req, res) => {
  res.status(404).send('Not found');
});

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

A Because the 404 handler is placed before the route handlers.

B Because the route '/hello' matches the request, so 404 handler doesn't run for that path.

C Because the 404 handler is missing the error parameter, so it is treated as normal middleware.

D Because the 404 handler does not call next(), so it blocks other middleware.

💡 Hint

Think about which routes match requests and middleware order.

The route '/hello' matches requests to '/hello' and sends a response, so the request never reaches the 404 handler middleware.

❓ state_output

advanced

What is the response status and body for this 404 handler with next() call?

Analyze this Express 404 handler. What will the client receive?

Express

app.use((req, res, next) => {
  res.status(404);
  next();
});

app.use((req, res) => {
  res.send('Page missing');
});

A Status 404 with empty body.

B Status 200 with body 'Page missing'.

C Status 404 with body 'Page missing'.

D Status 200 with empty body.

💡 Hint

Check how status and next() affect response.

The first middleware sets status 404 and calls next(), so the next middleware sends the body with status 404 preserved.

🧠 Conceptual

expert

Which statement about Express 404 handlers is true?

Select the correct statement about how Express handles 404 Not Found errors.

A A 404 handler should be placed after all route handlers and middleware to catch unmatched requests.

B Express automatically sends a 404 response if no middleware handles a request, so no 404 handler is needed.

C A 404 handler must have four parameters (err, req, res, next) to catch unmatched routes.

D A 404 handler can only send JSON responses, not plain text or HTML.

💡 Hint

Think about middleware order and parameters.

404 handlers catch unmatched routes and must be last. Error handlers have four parameters; 404 handlers usually have three. Express does not auto-send 404 responses. 404 handlers can send any content type.