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.