req.headers for HTTP headers in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does req.headers contain in Express?

In an Express app, what kind of data does req.headers hold?

Express

app.get('/', (req, res) => {
  res.json(req.headers);
});

A An array of header names only.

B A string containing all headers concatenated with commas.

C An object with all HTTP headers sent by the client, with header names as keys and their values as strings.

D A function to retrieve headers by name.

💡 Hint

Think about how HTTP headers are structured and how Express exposes them.

req.headers is a plain JavaScript object where each key is a header name in lowercase, and the value is the header's value as a string.

📝 Syntax

intermediate

Accessing a specific header value from req.headers

Which code correctly gets the value of the content-type header from req.headers in Express?

Express

app.post('/data', (req, res) => {
  const contentType = /* fill here */;
  res.send(contentType);
});

A req.headers['content-type']

B req.headers['Content-Type']

C req.headers.contentType

D req.get('content-type')

💡 Hint

Header names in req.headers are all lowercase.

Express stores header names in lowercase in req.headers, so you must use lowercase keys to access them.

🔧 Debug

advanced

Why does req.headers['Authorization'] return undefined?

Given this code snippet, why might req.headers['Authorization'] be undefined?

app.get('/secret', (req, res) => {
  const auth = req.headers['Authorization'];
  res.send(auth || 'No auth');
});

A Header names in <code>req.headers</code> are lowercase, so it should be <code>req.headers['authorization']</code>.

B The client did not send the Authorization header, so it is undefined.

C Express blocks the Authorization header by default for security.

D You must use <code>req.get('Authorization')</code> instead of <code>req.headers</code>.

💡 Hint

Check how Express normalizes header names in req.headers.

Express converts all header names to lowercase in req.headers. Accessing with uppercase letters returns undefined.

❓ state_output

advanced

What is the output of this Express route logging req.headers?

Consider this Express route:

app.get('/test', (req, res) => {
  console.log(req.headers);
  res.send('ok');
});

If a client sends a request with headers:

{
  'Host': 'example.com',
  'User-Agent': 'curl/7.68.0',
  'Accept': '*/*'
}

What will be logged to the console?

A 'Host: example.com, User-Agent: curl/7.68.0, Accept: */*'

B { Host: 'example.com', User-Agent: 'curl/7.68.0', Accept: '*/*' }

C [ 'Host', 'User-Agent', 'Accept' ]

D { host: 'example.com', 'user-agent': 'curl/7.68.0', accept: '*/*' }

💡 Hint

Remember how Express formats header names in req.headers.

Express lowercases all header names in req.headers and stores them as keys in an object.

🧠 Conceptual

expert

Why should you avoid modifying req.headers directly in Express?

In Express, what is the main reason you should not change req.headers directly inside a middleware or route handler?

A Because <code>req.headers</code> is a read-only object and modifying it throws an error.

B Because modifying <code>req.headers</code> can cause unexpected behavior in downstream middleware or routing logic that rely on original headers.

C Because Express automatically resets <code>req.headers</code> after each middleware.

D Because headers must be modified only on the response object, not the request.

💡 Hint

Think about how middleware and routing depend on request data.

Changing req.headers can confuse later middleware or route handlers that expect the original headers, leading to bugs or security issues.

Practice

1. What does req.headers represent in an Express app?

easy

A. A function to send headers to the client

B. An object containing all HTTP headers sent by the client

C. A string with the URL path of the request

D. A number representing the HTTP status code

Solution

1. Step 1: Understand the role of req.headers

   req.headers holds all headers sent by the client in the HTTP request.

2. Step 2: Identify what req.headers contains

   It is an object with keys as header names (lowercase) and values as header values.

3. Final Answer:

   An object containing all HTTP headers sent by the client -> Option B

4. Quick Check:

   req.headers = client headers [OK]

Hint: Remember: req.headers holds client headers as an object [OK]

Common Mistakes:

• Thinking req.headers is a function
• Confusing headers with URL or status code
• Expecting headers keys to be uppercase

2. Which of the following is the correct way to access the 'content-type' header from req.headers?

easy

A. req.headers.Content-Type

B. req.headers['Content-Type']

C. req.headers.contentType

D. req.headers['content-type']

Solution

1. Step 1: Recall header key casing in req.headers

   Headers keys are always lowercase in req.headers.

2. Step 2: Choose the correct syntax to access the header

   Use bracket notation with lowercase key: req.headers['content-type'].

3. Final Answer:

   req.headers['content-type'] -> Option D

4. Quick Check:

   Headers keys are lowercase strings [OK]

Hint: Use lowercase keys with bracket notation for headers [OK]

Common Mistakes:

• Using camelCase or uppercase keys
• Using dot notation with hyphenated keys
• Assuming headers keys keep original case

3. Given this Express code snippet:

app.get('/', (req, res) => {
  const userAgent = req.headers['user-agent'];
  res.send(userAgent);
});

What will the server send back to the client?

medium

A. An error because 'user-agent' is not a valid header

B. The server's operating system name

C. The client's browser user agent string

D. An empty string

Solution

1. Step 1: Identify what req.headers['user-agent'] contains

   This header holds the client's browser and system info string.

2. Step 2: Understand the response sent

   The server sends back this string as the response body using res.send().

3. Final Answer:

   The client's browser user agent string -> Option C

4. Quick Check:

   req.headers['user-agent'] = client info [OK]

Hint: User-Agent header shows client browser info [OK]

Common Mistakes:

• Thinking it returns server info
• Assuming 'user-agent' header is invalid
• Expecting an error or empty response

4. What is wrong with this code snippet?

app.get('/data', (req, res) => {
  const auth = req.headers.Authorization;
  if (!auth) {
    res.status(401).send('Unauthorized');
  } else {
    res.send('Access granted');
  }
});

medium

A. Header keys in req.headers are lowercase, so 'Authorization' should be 'authorization'

B. Header keys in req.headers are case-sensitive and 'Authorization' is correct

C. You cannot access headers directly from req.headers

D. res.status() cannot be chained with res.send()

Solution

1. Step 1: Check header key casing in req.headers

   All header keys are lowercase, so 'Authorization' is incorrect.

2. Step 2: Correct the header key usage

   Use req.headers.authorization to access the Authorization header properly.

3. Final Answer:

   Header keys in req.headers are lowercase, so 'Authorization' should be 'authorization' -> Option A

4. Quick Check:

   Headers keys are lowercase strings [OK]

Hint: Always use lowercase keys for req.headers access [OK]

Common Mistakes:

• Using uppercase or camelCase header keys
• Thinking headers are case-sensitive as sent
• Believing res.status() and res.send() can't chain

5. You want to check if a client sent a custom header named 'X-Client-Id' and respond with 'Client ID: <id>'. Which code correctly handles this in Express?

hard

A. const clientId = req.headers['x-client-id']; if(clientId) res.send(`Client ID: ${clientId}`); else res.send('No Client ID');

B. const clientId = req.headers.xClientId; if(clientId) res.send(`Client ID: ${clientId}`); else res.send('No Client ID');

C. const clientId = req.headers['X-Client-Id']; if(clientId) res.send(`Client ID: ${clientId}`); else res.send('No Client ID');

D. const clientId = req.headers.get('X-Client-Id'); if(clientId) res.send(`Client ID: ${clientId}`); else res.send('No Client ID');

Solution

1. Step 1: Understand header key casing in req.headers

   Headers keys are lowercase, so use req.headers['x-client-id'].

2. Step 2: Verify the correct way to access custom headers

   const clientId = req.headers['x-client-id']; if(clientId) res.send(`Client ID: ${clientId}`); else res.send('No Client ID'); uses lowercase key and bracket notation correctly to get the header value.

3. Step 3: Check other options

   const clientId = req.headers['X-Client-Id']; if(clientId) res.send(`Client ID: ${clientId}`); else res.send('No Client ID'); uses uppercase keys (wrong), B uses camelCase (wrong), D uses req.headers.get('X-Client-Id') which doesn't exist on req.headers.

4. Final Answer:

   const clientId = req.headers['x-client-id']; if(clientId) res.send(`Client ID: ${clientId}`); else res.send('No Client ID'); -> Option A

5. Quick Check:

   Headers keys lowercase in req.headers [OK]

Hint: Custom headers keys are lowercase in req.headers [OK]

Common Mistakes:

• Using uppercase or camelCase keys
• Confusing req.headers with req.get()
• Accessing headers with dot notation for hyphenated names