Error response formatting in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of formatting error responses in Express?

Formatting error responses helps clients understand what went wrong by providing clear, consistent messages and status codes.

beginner

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

The status code 500 is used for internal server errors indicating something went wrong on the server side.

beginner

How can you send a JSON error response with a message and status code in Express?

Use res.status(code).json({ error: 'message' }) to send a JSON error with the right HTTP status.

intermediate

Why should error responses include both a status code and a message?

Status codes tell the client the type of error, while messages explain details, making debugging easier.

intermediate

What is a common pattern to handle errors globally in an Express app?

Use an error-handling middleware function with four arguments (err, req, res, next) to catch and format errors consistently.

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

A res.status()

B res.sendStatus()

C res.json()

D res.setStatus()

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

What is the correct way to send a JSON error message with status 404 in Express?

A res.sendStatus(404, { error: 'Not found' })

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

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

D res.error(404, 'Not found')

res.status(404).json({ error: 'Not found' }) sets status and sends JSON error.

In Express, what does an error-handling middleware function require?

A No parameters

B Four parameters: err, req, res, next

C Two parameters: req, res

D Three parameters: req, res, next

Error middleware must have four parameters to catch errors properly.

Why is it important to send consistent error response formats?

A To help clients parse and handle errors easily

B To make the server faster

C To reduce the size of responses

D To avoid using status codes

Consistent formats help clients understand and react to errors correctly.

Which status code indicates a client error due to bad input?

A 500

B 302

C 200

D 400

400 means Bad Request, indicating client-side input errors.

Explain how to create a global error handler in Express and why it is useful.

Think about middleware that catches errors and sends a response.

Describe the key parts of a well-formatted error response in an Express app.

What does the client need to understand the error?

Practice

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

easy

A. To redirect the user to an error page

B. To send the error message as plain text

C. To set the HTTP status code of the response

D. To log the error on the server console

Solution

1. Step 1: Understand the role of res.status()

   This method sets the HTTP status code that tells the client what kind of response it is (success, error, etc.).

2. Step 2: Differentiate from other response methods

   Unlike sending messages or logging, res.status() only sets the status code, which is essential for error handling.

3. Final Answer:

   To set the HTTP status code of the response -> Option C

4. Quick Check:

   res.status() sets status code [OK]

Hint: Remember: status() sets code, json() sends data [OK]

Common Mistakes:

• Confusing status() with sending the error message
• Using status() to log errors instead of setting code
• Thinking status() redirects the user

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

easy

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

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

C. res.status(404).send('Not found')

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

Solution

1. Step 1: Identify correct chaining of methods

   In Express, res.status(code) sets the status, and res.json(object) sends JSON data.

2. Step 2: Check each option's syntax

   res.status(404).json({ error: 'Not found' }) correctly chains status(404) and json({ error: 'Not found' }). Others misuse method order or types.

3. Final Answer:

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

4. Quick Check:

   Use status() then json() [OK]

Hint: Set status first, then send JSON with json() [OK]

Common Mistakes:

• Calling json() before status()
• Using sendStatus() then json() which is invalid
• Sending plain text instead of JSON for errors

3. What will be the HTTP status code and JSON response body when this Express code runs?

app.get('/data', (req, res) => {
  if (!req.query.id) {
    return res.status(400).json({ error: 'ID is required' });
  }
  res.json({ id: req.query.id });
});

medium

A. Status 400 with body { error: 'ID is required' }

B. Status 200 with body { id: undefined }

C. Status 404 with body { error: 'Not found' }

D. Status 500 with empty body

Solution

1. Step 1: Analyze the condition for missing query parameter

   If req.query.id is missing, the code returns a 400 status with JSON error message.

2. Step 2: Understand the response when ID is missing

   The code sends res.status(400).json({ error: 'ID is required' }), so status is 400 and body contains the error message.

3. Final Answer:

   Status 400 with body { error: 'ID is required' } -> Option A

4. Quick Check:

   Missing id triggers 400 error JSON [OK]

Hint: Check conditions that trigger res.status() before sending response [OK]

Common Mistakes:

• Assuming status 200 when id is missing
• Confusing 400 with 404 status codes
• Ignoring the early return on error

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

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

medium

A. The status code is set after sending the response

B. The error message should not be sent in JSON

C. The middleware is missing the next() call

D. The error handler should not have four parameters

Solution

1. Step 1: Check order of response methods

   Calling res.json() sends the response immediately. Setting status after sending has no effect.

2. Step 2: Correct order for setting status and sending response

   Status must be set before sending JSON: res.status(500).json(...).

3. Final Answer:

   The status code is set after sending the response -> Option A

4. Quick Check:

   Status before sending response [OK]

Hint: Always set status before sending response [OK]

Common Mistakes:

• Setting status after res.json()
• Not sending status at all
• Misunderstanding middleware parameters

5. You want to send a 422 Unprocessable Entity error with a JSON body containing { error: 'Invalid input', fields: ['name', 'email'] }. Which Express code snippet correctly formats this error response?

hard

A. res.json({ error: 'Invalid input', fields: ['name', 'email'] }).status(422);

B. res.status(422).json({ error: 'Invalid input', fields: ['name', 'email'] });

C. res.sendStatus(422).json({ error: 'Invalid input', fields: ['name', 'email'] });

D. res.status(400).json({ error: 'Invalid input', fields: ['name', 'email'] });

Solution

1. Step 1: Choose the correct status code and method order

   Use res.status(422) to set the correct HTTP status code before sending JSON.

2. Step 2: Verify the JSON body matches the required error format

   The JSON object includes both error and fields keys as specified.

3. Final Answer:

   res.status(422).json({ error: 'Invalid input', fields: ['name', 'email'] }); -> Option B

4. Quick Check:

   Status 422 then json() with error details [OK]

Hint: Set status 422 first, then send JSON error object [OK]

Common Mistakes:

• Calling json() before status()
• Using wrong status code like 400
• Using sendStatus() which sends no JSON body