Error response formatting in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of error response formatting in Node.js APIs?

Error response formatting helps send clear, consistent messages to clients when something goes wrong. It makes debugging easier and improves user experience by providing understandable error details.

beginner

Which HTTP status code is commonly used for a generic server error?

The HTTP status code 500 is used for generic server errors, indicating that something went wrong on the server side.

intermediate

Why should error responses include a consistent JSON structure?

Consistent JSON structure allows clients to easily parse and handle errors programmatically. It also helps frontend developers display meaningful messages to users.

beginner

Show a simple example of a JSON error response format.

A simple JSON error response might look like: {"error": {"message": "Not Found", "code": 404}}. This clearly tells the client what went wrong and the error code.

intermediate

What is a good practice when sending error details in production environments?

In production, avoid sending sensitive error details to clients. Instead, send a generic message and log the detailed error on the server for developers to review.

Which HTTP status code indicates a client error due to bad request?

A 400

B 500

C 200

D 302

400 means Bad Request, indicating the client sent invalid data.

What is the main benefit of formatting error responses as JSON?

A It speeds up the server

B It makes errors easier to read and handle by clients

C It hides all error details

D It changes the HTTP status code automatically

JSON formatting helps clients parse and display errors clearly.

Which of these is NOT a good practice for error response formatting?

A Logging detailed errors on the server

B Sending consistent error structure

C Using proper HTTP status codes

D Including sensitive server details in production errors

Avoid exposing sensitive details to clients in production.

What should an error response include besides the HTTP status code?

A A clear error message

B The full server stack trace

C Client's IP address

D User's password

A clear message helps clients understand the error.

In Node.js, which method is commonly used to send a JSON error response?

A console.log()

B res.sendFile()

C res.json()

D process.exit()

res.json() sends a JSON response to the client.

Explain why consistent error response formatting is important in Node.js APIs.

Think about how clients and developers benefit from predictable error messages.

Describe best practices for sending error responses in production environments.

Consider security and user experience.

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