Error response formatting in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Express error handler?

Consider this Express.js error handling middleware. What JSON response will the client receive if an error with message 'Invalid input' and status 400 is passed?

Node.js

app.use((err, req, res, next) => {
  res.status(err.status || 500).json({
    error: {
      message: err.message || 'Internal Server Error',
      code: err.status || 500
    }
  });
});

A {"error":{"message":"Internal Server Error","code":500}}

B {"error":{"msg":"Invalid input","status":400}}

C {"message":"Invalid input","code":400}

D {"error":{"message":"Invalid input","code":400}}

💡 Hint

Look at how the error message and status are accessed and structured in the JSON response.

The middleware sends a JSON object with an 'error' key containing 'message' and 'code' properties. It uses the error's message and status if provided, otherwise defaults.

❓ Predict Output

intermediate

What error does this Express error handler cause?

What error will this Express error handler cause when an error is passed to it?

Node.js

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

A No error, sends JSON with error message and undefined status code

B TypeError: Cannot read property 'status' of undefined

C ReferenceError: err is not defined

D SyntaxError: Unexpected token

💡 Hint

Consider what happens if err.status is undefined.

If err.status is undefined, res.status(undefined) defaults to 200 OK in Express, so no error is thrown. The JSON contains the error message but status code is 200.

❓ component_behavior

advanced

Which option correctly formats a JSON error response with stack trace only in development?

You want to send an error response JSON that includes the error message always, but includes the stack trace only if NODE_ENV is 'development'. Which code snippet does this correctly?

A res.status(err.status || 500).json({ message: err.message });

B res.status(err.status || 500).json({ message: err.message, stack: process.env.NODE_ENV === 'production' ? err.stack : undefined });

C res.status(err.status || 500).json({ message: err.message, stack: process.env.NODE_ENV === 'development' ? err.stack : undefined });

D res.status(err.status || 500).json({ message: err.message, stack: err.stack });

💡 Hint

Check the environment variable condition for including the stack trace.

Option C includes the stack trace only if NODE_ENV is 'development', which is the common practice to avoid exposing stack traces in production.

📝 Syntax

advanced

Which option causes a syntax error in this error response code?

Identify the option that will cause a syntax error when used as an Express error handler response.

Node.js

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

A res.status(500).json({ error: err.message, details: err.stack, })

B res.status(500).json({ error: err.message details: err.stack })

C res.status(500).json({ error: err.message, details: err.stack })

D res.status(500).json({ error: err.message + details: err.stack })

💡 Hint

Look for missing commas or invalid object syntax.

Option B misses a comma between properties, causing a syntax error.

🔧 Debug

expert

Why does this Express error handler send a 200 status instead of 500?

Given this error handler, why does the client receive a 200 OK status instead of 500 when an error occurs? app.use((err, req, res, next) => { res.status(err.status).json({ error: err.message }); });

A Because err.status is undefined, res.status(undefined) defaults to 200 OK in Express

B Because the error handler is missing next() call to propagate the error

C Because res.status() always sends 200 unless explicitly called with 500

D Because JSON responses always override status codes to 200

💡 Hint

Check what happens if err.status is not set.

If err.status is undefined, Express treats res.status(undefined) as res.status(200), so the response status is 200 OK instead of an error code.

Practice

1. What is the main purpose of using res.status() in Node.js error handling?

easy

A. To log errors on the server console

B. To send a success message to the client

C. To restart the server automatically

D. To set the HTTP status code for the response

Solution

1. Step 1: Understand HTTP status codes

   HTTP status codes tell the client if a request succeeded or failed.

2. Step 2: Role of res.status()

   This method sets the HTTP status code in the response, indicating success or error.

3. Final Answer:

   To set the HTTP status code for the response -> Option D

4. Quick Check:

   res.status() sets HTTP status code [OK]

Hint: Remember: status sets code, not message or logs [OK]

Common Mistakes:

• Confusing status with sending messages
• Thinking it logs errors
• Assuming it restarts the server

2. Which of the following is the correct way to send a JSON error response with status 404 in Express?

easy

A. res.send(404).json({ error: 'Not found' })

B. res.status(404).json({ error: 'Not found' })

C. res.json(404, { error: 'Not found' })

D. res.error(404).send({ error: 'Not found' })

Solution

1. Step 1: Check Express response methods

   Use res.status(code) to set status, then json() to send JSON.

2. Step 2: Validate syntax correctness

   Only res.status(404).json({ error: 'Not found' }) is valid syntax.

3. Final Answer:

   res.status(404).json({ error: 'Not found' }) -> Option B

4. Quick Check:

   Set status then send JSON [OK]

Hint: Use res.status(code).json(object) for error responses [OK]

Common Mistakes:

• Calling res.send() before json()
• Passing status code inside json()
• Using non-existent res.error() method

3. Given this Express error handler code:

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

What will the client receive if an error with message 'Database down' occurs?

medium

A. {"message":"Database down","code":"SERVER_ERROR"} with status 500

B. {"error":"Database down"} with status 200

C. Plain text 'Database down' with status 500

D. Empty response with status 500

Solution

1. Step 1: Analyze the error handler response

   The handler sets status 500 and sends JSON with message and code fields.

2. Step 2: Match error message and response

   The JSON includes the error message 'Database down' and code 'SERVER_ERROR'.

3. Final Answer:

   {"message":"Database down","code":"SERVER_ERROR"} with status 500 -> Option A

4. Quick Check:

   Error message and code sent as JSON with 500 status [OK]

Hint: Error handlers send JSON with message and code [OK]

Common Mistakes:

• Assuming status 200 by default
• Expecting plain text instead of JSON
• Missing error code in response

4. Identify the error in this Express error response code:

app.use((err, req, res) => {
  res.status(400).json({ message: err.message });
});

medium

A. Missing the next parameter in the middleware function

B. Using res.status() instead of res.sendStatus()

C. Not calling res.end() after json()

D. Incorrect status code for errors

Solution

1. Step 1: Check Express error middleware signature

   Error middleware must have four parameters: err, req, res, next.

2. Step 2: Identify missing parameter

   The code has only three parameters, missing next, so Express won't recognize it as error handler.

3. Final Answer:

   Missing the next parameter in the middleware function -> Option A

4. Quick Check:

   Error middleware needs 4 params: err, req, res, next [OK]

Hint: Error middleware must have 4 parameters [OK]

Common Mistakes:

• Leaving out next parameter
• Confusing res.status() with res.sendStatus()
• Thinking json() needs res.end()

5. You want to send a detailed error response with a custom code and message, but only if the error has a statusCode property. Otherwise, send a 500 status with a generic message. Which code snippet correctly implements this in Express?

hard

A. app.use((err, req, res, next) => {
if (err.statusCode) {
res.json({ code: err.statusCode, message: err.message });
} else {
res.status(500).send('Error');
}
});

B. app.use((err, req, res) => {
res.status(err.statusCode).json({ message: err.message });
});

C. app.use((err, req, res, next) => {
const status = err.statusCode || 500;
const message = err.message || 'Internal Server Error';
res.status(status).json({ code: status, message });
});

D. app.use((err, req, res, next) => {
res.status(500).json({ code: 'ERROR', message: 'Something went wrong' });
});

Solution

1. Step 1: Check for error statusCode and message

   Use err.statusCode if present, else default to 500; similarly for message.

2. Step 2: Send JSON response with status and message

   Set status with res.status(status) and send JSON with code and message fields.

3. Final Answer:

   app.use((err, req, res, next) => { const status = err.statusCode || 500; const message = err.message || 'Internal Server Error'; res.status(status).json({ code: status, message }); }); -> Option C

4. Quick Check:

   Use defaults and send JSON with status and message [OK]

Hint: Use || to default status and message in error handler [OK]

Common Mistakes:

• Missing next parameter in middleware
• Not defaulting status or message
• Sending plain text instead of JSON