Error response formatting in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to send a JSON error response with status 404.

Express

app.get('/item/:id', (req, res) => {
  const item = database.find(i => i.id === req.params.id);
  if (!item) {
    return res.status([1]).json({ error: 'Item not found' });
  }
  res.json(item);
});

Drag options to blanks, or click blank then click option'

A200

B500

C404

D400

Attempts:

3 left

💡 Hint

Common Mistakes

Using status 200 for errors

Using status 500 for client errors

Use status code 404 to indicate the requested item was not found.

2fill in blank

medium

Complete the code to send a JSON error response with a custom message and status 400.

Express

app.post('/user', (req, res) => {
  if (!req.body.name) {
    return res.status([1]).json({ error: 'Name is required' });
  }
  res.json({ success: true });
});

Drag options to blanks, or click blank then click option'

A401

B404

C403

D400

Attempts:

3 left

💡 Hint

Common Mistakes

Using 401 which means unauthorized

Using 404 which means not found

Status 400 means 'Bad Request', suitable when required data is missing.

3fill in blank

hard

Fix the error in the code to correctly send a JSON error response with status 500.

Express

app.get('/data', (req, res) => {
  try {
    const data = getData();
    res.json(data);
  } catch (error) {
    res.status([1]).json({ error: error.message });
  }
});

Drag options to blanks, or click blank then click option'

A500

B400

C404

D200

Attempts:

3 left

💡 Hint

Common Mistakes

Using res.send instead of res.json for JSON response

Using wrong status codes like 200 or 404

Use status 500 for server errors and send JSON with res.status(500).json(...).

4fill in blank

hard

Fill both blanks to send a JSON error response with status 401 and a message.

Express

app.use((req, res) => {
  res.status([1]).json({
    error: [2]
  });
});

Drag options to blanks, or click blank then click option'

A401

B'Unauthorized access'

C'Not Found'

D403

Attempts:

3 left

💡 Hint

Common Mistakes

Using status 403 which means forbidden

Using error message 'Not Found' for unauthorized

Status 401 means unauthorized. The error message should explain the issue.

5fill in blank

hard

Fill all three blanks to send a JSON error response with status 403 and a detailed message.

Express

app.get('/admin', (req, res) => {
  if (!req.user || !req.user.isAdmin) {
    return res.status([1]).json({
      error: [2],
      message: [3]
    });
  }
  res.json({ secret: '42' });
});

Drag options to blanks, or click blank then click option'

A401

B'Forbidden'

C'You do not have permission to access this resource.'

D403

Attempts:

3 left

💡 Hint

Common Mistakes

Using 401 instead of 403 for forbidden

Using vague or missing error messages

Status 403 means forbidden. The error and message explain the access denial.

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