404 Not Found handler in Express - Cheat Sheet & Quick Revision

Recall & Review

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What is a 404 Not Found handler in Express?

It is a middleware function that catches requests to routes that do not exist and sends a 404 status with a message to the client.

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Where should you place the 404 Not Found handler in your Express app?

It should be placed after all other route handlers so it only runs if no route matches the request.

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How do you send a 404 status code in Express?

Use res.status(404).send('Not Found') or res.status(404).json({ message: 'Not Found' }) inside the handler.

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Example code snippet for a simple 404 Not Found handler in Express?

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

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Why is it important to have a 404 Not Found handler in your Express app?

It improves user experience by clearly telling users when a page or resource does not exist, instead of leaving them with a blank or confusing error.

Where should the 404 Not Found handler be placed in an Express app?

A It does not matter

B Before all route handlers

C In the middle of route handlers

D After all other route handlers

The 404 handler runs only if no other route matches, so it must be last.

What status code does a 404 Not Found handler send?

A 404

B 200

C 500

D 302

404 means the requested resource was not found.

Which Express method sets the HTTP status code in the response?

A res.status()

B res.setStatus()

C res.sendStatus()

D res.code()

res.status() sets the status code before sending the response.

What happens if you do not add a 404 handler in Express?

A Express automatically sends a 404 response

B The server crashes

C The request hangs or returns a default response

D The request is redirected

Without a 404 handler, requests to unknown routes may hang or get a default empty response.

How do you define a 404 handler middleware in Express?

A app.get('/404', ...)

B app.use((req, res) => { ... })

C app.post('/404', ...)

D app.route('/404')

A 404 handler uses app.use with a function that handles unmatched requests.

Explain how to create and use a 404 Not Found handler in an Express app.

Think about middleware order and response status

Why is a 404 Not Found handler important for user experience in web apps?

Consider what happens if users get no feedback