Error response formatting in Node.js - Mini Project: Build & Apply

Error Response Formatting in Node.js

📖 Scenario: You are building a simple Node.js server that sends error responses in a consistent JSON format. This helps clients understand what went wrong.

🎯 Goal: Create a Node.js server that returns error responses with a JSON object containing status, message, and errorCode fields.

📋 What You'll Learn

Create an error object with specific properties

Add a configuration variable for default error code

Write a function to format the error response JSON

Send the formatted error response in the server

💡 Why This Matters

🌍 Real World

APIs and web servers need to send clear, consistent error messages so clients can handle problems properly.

💼 Career

Understanding error response formatting is essential for backend developers working with Node.js and building reliable web services.

1

Create the error object

Create a constant called error with these exact properties: status set to 400, message set to "Invalid input", and errorCode set to "INVALID_INPUT".

Node.js

// Your code here

Hint

Use const error = { status: 400, message: "Invalid input", errorCode: "INVALID_INPUT" }.

2

Add default error code configuration

Create a constant called DEFAULT_ERROR_CODE and set it to the string "UNKNOWN_ERROR".

Node.js

const error = {
  status: 400,
  message: "Invalid input",
  errorCode: "INVALID_INPUT"
};

// Your code here

Hint

Use const DEFAULT_ERROR_CODE = "UNKNOWN_ERROR"; to set the default error code.

3

Write the error formatting function

Write a function called formatErrorResponse that takes an error object as input and returns a new object with status, message, and errorCode. Use the errorCode from the input error if it exists; otherwise, use DEFAULT_ERROR_CODE.

Node.js

const error = {
  status: 400,
  message: "Invalid input",
  errorCode: "INVALID_INPUT"
};

const DEFAULT_ERROR_CODE = "UNKNOWN_ERROR";

// Your code here

Hint

Use error.errorCode || DEFAULT_ERROR_CODE to provide a fallback error code.

4

Send the formatted error response in a Node.js server

Create a simple Node.js HTTP server using http.createServer. In the request handler, use formatErrorResponse(error) to get the error JSON, set the response header Content-Type to application/json, and send the JSON stringified error response with the status code from error.status.

Node.js

const error = {
  status: 400,
  message: "Invalid input",
  errorCode: "INVALID_INPUT"
};

const DEFAULT_ERROR_CODE = "UNKNOWN_ERROR";

function formatErrorResponse(error) {
  return {
    status: error.status,
    message: error.message,
    errorCode: error.errorCode || DEFAULT_ERROR_CODE
  };
}

// Your code here

Hint

Use http.createServer and res.writeHead with Content-Type set to application/json.

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