HTTP caching headers (ETag, Cache-Control) in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the effect of setting Cache-Control: no-store in Express?

Consider an Express server that sets the header Cache-Control: no-store on responses. What will happen when a browser receives this header?

Express

app.get('/data', (req, res) => {
  res.set('Cache-Control', 'no-store');
  res.send('Fresh data');
});

A The browser will never cache the response and will always request fresh data.

B The browser will cache the response but revalidate it before each use.

C The browser will cache the response and use it without revalidation until it expires.

D The browser will cache the response only if the ETag matches.

💡 Hint

Think about what no-store means for caching.

Cache-Control: no-store tells browsers not to store any part of the response. This means the browser will always fetch fresh data and never use a cached copy.

📝 Syntax

intermediate

Which Express code correctly sets an ETag header?

Which code snippet correctly sets a strong ETag header with value "12345" on the response?

A res.header('etag', 'W/12345');

B res.set('ETag', '"12345"');

C res.set('ETag', 'W/"12345"');

D res.setHeader('etag', '12345');

💡 Hint

Strong ETags do not start with W/.

A strong ETag is a quoted string without the W/ prefix. Option B sets ETag to "12345" correctly. Option B sets a weak ETag. Option B uses lowercase header name which Express normalizes but lacks quotes. Option B is missing quotes and uses lowercase header name.

❓ state_output

advanced

What is the response status when If-None-Match matches ETag?

An Express server sends ETag: "abc123". The client sends a request with header If-None-Match: "abc123". What status code will the server respond with if it uses res.status(304).end() on match?

Express

app.get('/resource', (req, res) => {
  const etag = '"abc123"';
  res.set('ETag', etag);
  if (req.headers['if-none-match'] === etag) {
    res.status(304).end();
  } else {
    res.send('Resource content');
  }
});

A 200 OK with resource content

B 500 Internal Server Error

C 404 Not Found

D 304 Not Modified with no body

💡 Hint

What does status 304 mean in HTTP?

Status 304 means the resource has not changed. The server tells the client to use its cached copy without sending the body again.

🔧 Debug

advanced

Why does this Express code not cache responses as expected?

Given this Express code, why might the browser still cache responses despite setting Cache-Control: no-cache?

Express

app.get('/info', (req, res) => {
  res.set('Cache-Control', 'no-cache');
  res.send('Info data');
});

A Because <code>no-cache</code> means the browser must revalidate but can cache the response.

B Because <code>no-cache</code> disables caching completely.

C Because the server forgot to set <code>ETag</code> header.

D Because <code>Cache-Control</code> header is ignored by browsers.

💡 Hint

Check the meaning of no-cache in HTTP caching.

Cache-Control: no-cache means the browser can store the response but must check with the server before using it. It does not prevent caching entirely.

🧠 Conceptual

expert

How does Express handle ETag generation by default?

Which statement best describes how Express generates ETags for responses by default?

A Express automatically generates strong ETags based on response body content using a hash function.

B Express disables ETag generation by default; developers must set them manually.

C Express generates weak ETags by default using a hash of the response body.

D Express generates weak ETags by default using response content length and modification time.

💡 Hint

Check Express documentation on default ETag behavior.

Express generates weak ETags by default. It uses a hash of the response body to create a weak ETag, which starts with W/. This helps with caching but allows some flexibility in content changes.