Error response formatting in Node.js

Introduction

Error response formatting helps your app send clear and consistent messages when something goes wrong. It makes it easier for users and developers to understand the problem.

When your server needs to tell the client about a problem like missing data.

When you want to keep error messages consistent across your app.

When you want to include helpful details like error codes or suggestions.

When debugging issues by logging clear error information.

When building APIs that other programs will use and expect structured errors.

Syntax

Node.js

res.status(statusCode).json({ error: { message: 'Error message', code: 'ERROR_CODE' } })

Use res.status() to set the HTTP status code before sending the response.

Send error details inside a JSON object for easy reading and parsing.

Examples

Sends a 404 error with a message and code.

Node.js

res.status(404).json({ error: { message: 'Not found', code: 'NOT_FOUND' } })

Sends a 400 error when user input is wrong.

Node.js

res.status(400).json({ error: { message: 'Invalid input', code: 'BAD_REQUEST' } })

Sends a 500 error for unexpected server problems.

Node.js

res.status(500).json({ error: { message: 'Server error', code: 'INTERNAL_ERROR' } })

Sample Program

This Express app checks if the user ID is a number. If not, it sends a 400 error with a clear message. If the user ID is not '123', it sends a 404 error. Otherwise, it returns user data.

Node.js

import express from 'express';
const app = express();

app.get('/user/:id', (req, res) => {
  const userId = req.params.id;
  if (!userId.match(/^\d+$/)) {
    return res.status(400).json({ error: { message: 'User ID must be a number', code: 'INVALID_ID' } });
  }
  // Simulate user not found
  if (userId !== '123') {
    return res.status(404).json({ error: { message: 'User not found', code: 'USER_NOT_FOUND' } });
  }
  res.json({ id: userId, name: 'Alice' });
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Always set the correct HTTP status code to match the error type.

Keep error messages simple and helpful for users and developers.

Use consistent error codes to make it easier to handle errors in client apps.

Summary

Format error responses as JSON with a clear message and code.

Use HTTP status codes to indicate the type of error.

Consistent error formatting improves user experience and debugging.

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