Method chaining on response in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express response chaining?

Consider this Express route handler code. What will the client receive as the response body?

Express

app.get('/test', (req, res) => {
  res.status(201).type('text/plain').send('Success');
});

A The client receives a plain text response with status 201 and body 'Success'.

B The client receives a JSON response with status 200 and body 'Success'.

C The client receives a plain text response with status 200 and body 'Success'.

D The client receives a JSON response with status 201 and body 'Success'.

💡 Hint

Remember that method chaining sets status, content type, and then sends the body.

The status(201) sets the HTTP status code to 201. The type('text/plain') sets the Content-Type header to plain text. Finally, send('Success') sends the string as the response body. So the client gets status 201, plain text content type, and body 'Success'.

📝 Syntax

intermediate

Which option correctly chains methods to send JSON with status 404?

Choose the correct Express response chaining to send a JSON error message with status 404.

A res.status(404).sendJson({ error: 'Not found' });

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

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

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

💡 Hint

The status must be set before sending the response.

Method chaining requires setting the status before sending the response. res.status(404).json(...) correctly sets status 404 and sends JSON. Option D tries to set status after sending, which won't work. Option D uses send which sends JSON but chaining status after send is invalid. Option D uses a non-existent method sendJson.

🔧 Debug

advanced

Why does this chained response code cause an error?

Examine the code below. Why will it cause an error or unexpected behavior?

Express

app.get('/error', (req, res) => {
  res.send('First response').status(200);
});

A Because status(200) is invalid and should be statusCode(200).

B Because send cannot be called without a JSON object.

C Because status must be set before send; chaining status after send does nothing or errors.

D Because res.send returns undefined and chaining is not allowed.

💡 Hint

Think about the order of method calls and when headers are sent.

Calling res.send() sends the response immediately, including headers. Trying to set status after sending has no effect or may cause an error because headers are already sent. The correct order is to set status first, then send.

❓ state_output

advanced

What is the final status code sent to the client?

Given this Express code, what status code will the client receive?

Express

app.get('/chain', (req, res) => {
  res.status(202).status(204).send('Done');
});

A 202

B 204

C 200

D Error thrown due to multiple status calls

💡 Hint

Later status calls overwrite earlier ones.

Calling res.status(202) sets status to 202, but then res.status(204) overwrites it to 204. The last status call before send determines the final status code.

🧠 Conceptual

expert

Which statement about Express response method chaining is TRUE?

Select the true statement about method chaining on Express response objects.

A Calling res.send() ends the response and further chaining methods have no effect.

B You can chain res.send() multiple times to send multiple responses.

C res.json() cannot be chained with res.status() because they return different objects.

D res.status() must always be called after res.send() to set the status code correctly.

💡 Hint

Think about when the response is finalized.

Once res.send() is called, the response is sent and closed. Any further method calls on res do not affect the response. You cannot send multiple responses for one request. res.status() must be called before res.send(). Both res.status() and res.json() return the response object, so chaining is possible.

Practice

1. What does method chaining on the Express res object allow you to do?

easy

A. Combine multiple response methods in one line for cleaner code

B. Run multiple server instances simultaneously

C. Chain database queries with response methods

D. Automatically restart the server on errors

Solution

1. Step 1: Understand method chaining concept

   Method chaining means calling multiple methods one after another on the same object.

2. Step 2: Apply to Express response object

   Express res allows chaining methods like status(), set(), and send() to write concise code.

3. Final Answer:

   Combine multiple response methods in one line for cleaner code -> Option A

4. Quick Check:

   Method chaining = Combine methods [OK]

Hint: Think of chaining like linking commands in one line [OK]

Common Mistakes:

• Confusing method chaining with running multiple servers
• Mixing database queries with response methods
• Assuming chaining restarts the server

2. Which of the following is the correct way to chain methods on the Express res object?

easy

A. res.set('Content-Type', 'text/plain').status(200).send('OK')

B. res.status(200).send('OK').set('Content-Type', 'text/plain')

C. res.send('OK').status(200).set('Content-Type', 'text/plain')

D. res.status(200); res.send('OK'); res.set('Content-Type', 'text/plain')

Solution

1. Step 1: Check method chaining order

   Methods like status() and set() return the res object, allowing chaining before send().

2. Step 2: Identify correct chain ending

   The chain must end with send() to send the response and finish the chain.

3. Final Answer:

   res.set('Content-Type', 'text/plain').status(200).send('OK') -> Option A

4. Quick Check:

   Chain ends with send() [OK]

Hint: Always end chain with send() or end() [OK]

Common Mistakes:

• Calling send() before setting status or headers
• Trying to chain after send()
• Using semicolons instead of chaining dots

3. What will be the output of this Express code snippet?

app.get('/', (req, res) => {
  res.status(404).set('Content-Type', 'text/plain').send('Not Found');
});

medium

A. Response with status 200 and body 'Not Found'

B. Syntax error due to incorrect chaining

C. Response with status 404 and empty body

D. Response with status 404 and body 'Not Found' with plain text content type

Solution

1. Step 1: Analyze method chain

   The code sets status to 404, sets content type header, then sends 'Not Found' as body.

2. Step 2: Understand response behavior

   The response will have status 404, header 'Content-Type: text/plain', and body 'Not Found'.

3. Final Answer:

   Response with status 404 and body 'Not Found' with plain text content type -> Option D

4. Quick Check:

   Status 404 + send body 'Not Found' [OK]

Hint: Status and headers set before send() affect response [OK]

Common Mistakes:

• Assuming default status 200 is sent
• Thinking body is empty
• Believing chaining causes syntax error

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

res.send('Hello').status(200).set('Content-Type', 'text/html');

medium

A. No error, code works fine

B. Cannot chain methods after send() because response is already sent

C. Missing semicolon after send()

D. set() method does not exist on res object

Solution

1. Step 1: Understand send() behavior

   Calling send() sends the response and ends it, so chaining after it is invalid.

2. Step 2: Identify chaining error

   Methods status() and set() cannot be called after send() because response is finished.

3. Final Answer:

   Cannot chain methods after send() because response is already sent -> Option B

4. Quick Check:

   send() ends response, no chaining after [OK]

Hint: send() ends response; no chaining after it [OK]

Common Mistakes:

• Thinking chaining after send() is allowed
• Confusing semicolon placement with chaining error
• Believing set() is not a valid method

5. You want to send a JSON response with status 201 and a custom header X-Custom set to Success. Which of the following chained methods is correct?

hard

A. res.status(201).send({ message: 'Created' }).set('X-Custom', 'Success');

B. res.status(201).json({ message: 'Created' }); res.set('X-Custom', 'Success');

C. res.set('X-Custom', 'Success').status(201).json({ message: 'Created' });

D. res.json({ message: 'Created' }).status(201).set('X-Custom', 'Success');

Solution

1. Step 1: Order methods correctly

   Set headers first, then status, then send JSON with json() method.

2. Step 2: Avoid chaining after response sent

   Methods after json() or send() won't work because response ends.

3. Final Answer:

   res.set('X-Custom', 'Success').status(201).json({ message: 'Created' }); -> Option C

4. Quick Check:

   Headers + status + json() in chain [OK]

Hint: Set headers and status before json() or send() [OK]

Common Mistakes:

• Chaining set() after send() or json()
• Sending response before setting headers
• Using multiple separate statements instead of chaining