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.