res.set for response headers in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Express code send as response headers?

Consider this Express route handler:

app.get('/test', (req, res) => {
  res.set('Content-Type', 'application/json');
  res.set('X-Custom-Header', '12345');
  res.send('{}');
});

What headers will the client receive?

Express

app.get('/test', (req, res) => {
  res.set('Content-Type', 'application/json');
  res.set('X-Custom-Header', '12345');
  res.send('{}');
});

A No headers set, default Content-Type is text/html

B Only Content-Type: application/json

C Only X-Custom-Header: 12345

D Content-Type: application/json and X-Custom-Header: 12345

💡 Hint

res.set sets headers before sending the response.

The res.set method sets the specified headers. Here, two headers are set before sending the response, so both appear in the response headers.

📝 Syntax

intermediate

Which option correctly sets multiple headers using res.set?

You want to set Content-Type to 'text/plain' and Cache-Control to 'no-cache' in one call to res.set. Which code is correct?

A res.set(['Content-Type', 'text/plain'], ['Cache-Control', 'no-cache']);

B res.set({'Content-Type': 'text/plain', 'Cache-Control': 'no-cache'});

C res.set('Content-Type', 'text/plain', 'Cache-Control', 'no-cache');

D res.set('Content-Type: text/plain; Cache-Control: no-cache');

💡 Hint

res.set accepts an object to set multiple headers at once.

res.set accepts either a header name and value or an object with multiple headers. Only option B uses the correct object syntax.

🔧 Debug

advanced

Why does this code cause an error?

Look at this Express code snippet:

res.set();
res.send('Hello');

What error will occur and why?

Express

res.set();
res.send('Hello');

A ReferenceError because res is undefined

B SyntaxError due to missing colon in header string

C TypeError because res.set requires both header name and value

D No error, header is set with empty value

💡 Hint

Check the parameters required by res.set.

res.set requires either a header name and value or an object with multiple headers. Calling res.set() with no arguments causes a TypeError.

❓ state_output

advanced

What is the final value of the 'Content-Type' header?

Given this Express code:

res.set('Content-Type', 'text/html');
res.set('Content-Type', 'application/json');
res.send('{}');

What will be the Content-Type header sent to the client?

Express

res.set('Content-Type', 'text/html');
res.set('Content-Type', 'application/json');
res.send('{}');

A application/json

B text/html

C Both headers sent with different values

D No Content-Type header sent

💡 Hint

Later calls to res.set overwrite previous headers with the same name.

The second res.set call overwrites the first Content-Type header value. Only 'application/json' is sent.

🧠 Conceptual

expert

What happens if you call res.set after res.send?

In Express, what is the effect of calling res.set('X-Test', 'value') after res.send('done') has already been called?

Express

res.send('done');
res.set('X-Test', 'value');

A An error is thrown because headers cannot be set after sending

B The header is sent and overwrites previous headers

C The header is not sent because response is already finished

D Express queues the header and sends it after res.send

💡 Hint

Think about when headers are sent in the HTTP response lifecycle.

Once res.send is called, the response headers have already been sent to the client. Subsequent calls to res.set will throw an error because headers cannot be set after they are sent.

Practice

1. What does res.set do in an Express response?

easy

A. It sets or updates HTTP headers before sending the response.

B. It sends the response body to the client.

C. It closes the server connection immediately.

D. It logs request details to the console.

Solution

1. Step 1: Understand the purpose of res.set

   res.set is used to add or modify HTTP headers in the response before sending it.

2. Step 2: Differentiate from other response methods

   Sending the body is done by res.send, not res.set. Logging or closing connections are unrelated.

3. Final Answer:

   It sets or updates HTTP headers before sending the response. -> Option A

4. Quick Check:

   res.set sets headers [OK]

Hint: Remember: res.set changes headers, not body or connection [OK]

Common Mistakes:

• Confusing res.set with res.send
• Thinking res.set sends the response
• Assuming res.set closes the connection

2. Which of the following is the correct syntax to set a single header using res.set?

easy

A. res.set('Content-Type', application/json);

B. res.set('Content-Type', 'application/json');

C. res.set({Content-Type: 'application/json'});

D. res.set('Content-Type' => 'application/json');

Solution

1. Step 1: Recall correct method signature

   The correct syntax for setting one header is res.set(name, value) where both are strings.

2. Step 2: Check each option

   res.set('Content-Type', 'application/json'); uses correct syntax with two string arguments. res.set('Content-Type' => 'application/json'); uses invalid arrow syntax. res.set({Content-Type: 'application/json'}); uses invalid object syntax - unquoted key Content-Type (hyphen requires quotes). res.set('Content-Type', application/json); misses quotes around value.

3. Final Answer:

   res.set('Content-Type', 'application/json'); -> Option B

4. Quick Check:

   Two string args for single header [OK]

Hint: Use two strings for single header: name and value [OK]

Common Mistakes:

• Using arrow => instead of comma
• Passing unquoted values
• Confusing object syntax for single header

3. What will be the value of the Content-Type header after this code runs?

res.set({
  'Content-Type': 'text/html',
  'Cache-Control': 'no-cache'
});
res.set('Content-Type', 'application/json');

medium

A. Undefined (header not set)

B. 'text/html'

C. 'no-cache'

D. 'application/json'

Solution

1. Step 1: Understand header setting order

   First, Content-Type is set to 'text/html' via object. Then it is overwritten by res.set('Content-Type', 'application/json').

2. Step 2: Determine final header value

   The last set value for Content-Type is 'application/json', so that is the final header value.

3. Final Answer:

   'application/json' -> Option D

4. Quick Check:

   Last header set wins [OK]

Hint: Later res.set calls overwrite previous headers [OK]

Common Mistakes:

• Assuming first set value stays
• Confusing other headers with Content-Type
• Thinking headers merge values

4. Identify the error in this Express code snippet:

app.get('/data', (req, res) => {
  res.send('Hello');
  res.set('X-Custom-Header', '123');
});

medium

A. Headers must be set before sending the response body.

B. res.set cannot set custom headers.

C. res.send should be called twice to set headers.

D. No error; code works fine.

Solution

1. Step 1: Check order of res.set and res.send

   Headers must be set before calling res.send because sending the body finalizes the response.

2. Step 2: Identify the mistake

   Here, res.send is called first, then res.set, which is too late to add headers.

3. Final Answer:

   Headers must be set before sending the response body. -> Option A

4. Quick Check:

   Set headers before send [OK]

Hint: Always set headers before sending response body [OK]

Common Mistakes:

• Setting headers after res.send
• Thinking res.set can't set custom headers
• Calling res.send multiple times

5. You want to set multiple headers including Content-Type and Cache-Control in one call, but only if a condition is true. Which code correctly uses res.set with condition?

hard

A. res.set({'Content-Type': isJson ? 'application/json' : 'text/html'}); res.set('Cache-Control', 'no-store');

B. res.set('Content-Type', isJson ? 'application/json' : 'text/html', 'Cache-Control', 'no-store');

C. if (isJson) res.set({'Content-Type': 'application/json', 'Cache-Control': 'no-store'});

D. res.set('Content-Type', 'application/json' && 'Cache-Control', 'no-store');

Solution

1. Step 1: Understand how to set multiple headers conditionally

   Using an object with res.set allows setting many headers at once. Wrapping in an if statement applies condition.

2. Step 2: Evaluate each option

   if (isJson) res.set({'Content-Type': 'application/json', 'Cache-Control': 'no-store'}); correctly uses an if condition and sets multiple headers in one object. res.set('Content-Type', isJson ? 'application/json' : 'text/html', 'Cache-Control', 'no-store'); has invalid syntax with multiple pairs in one call. res.set({'Content-Type': isJson ? 'application/json' : 'text/html'}); res.set('Cache-Control', 'no-store'); splits headers into two calls but is valid; however, question asks for one call. res.set('Content-Type', 'application/json' && 'Cache-Control', 'no-store'); uses invalid logical operator in arguments.

3. Final Answer:

   if (isJson) res.set({'Content-Type': 'application/json', 'Cache-Control': 'no-store'}); -> Option C

4. Quick Check:

   if + res.set(object) for conditional multiple [OK]

Hint: Use object syntax inside if to set multiple headers [OK]

Common Mistakes:

• Passing multiple header pairs as separate arguments
• Using logical operators inside res.set arguments
• Setting headers outside condition