HTTP caching headers (ETag, Cache-Control) in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a Cache-Control header that tells browsers to cache the response for 1 hour.

Express

app.get('/data', (req, res) => {
  res.set('[1]', 'public, max-age=3600');
  res.send({ message: 'Hello' });
});

Drag options to blanks, or click blank then click option'

AExpires

BContent-Type

CETag

DCache-Control

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Content-Type' instead of 'Cache-Control'.

Confusing 'ETag' with cache duration headers.

The Cache-Control header controls how browsers cache responses. Setting it to public, max-age=3600 means the response can be cached for 3600 seconds (1 hour).

2fill in blank

medium

Complete the code to generate an ETag header using the express built-in method.

Express

app.get('/resource', (req, res) => {
  res.[1]();
  res.send('Resource content');
});

Drag options to blanks, or click blank then click option'

Aetag

BsetETag

CgenerateETag

DsetHeader

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use 'setHeader' directly without generating ETag.

Using non-existent methods like 'generateETag'.

Express has a built-in etag() method to generate and set the ETag header automatically.

3fill in blank

hard

Fix the error in the code to properly check the client's ETag and respond with 304 if it matches.

Express

app.get('/file', (req, res) => {
  const etag = '12345';
  if (req.headers['if-none-match'] === [1]) {
    res.status(304).end();
  } else {
    res.set('ETag', etag);
    res.send('File content');
  }
});

Drag options to blanks, or click blank then click option'

AETag

Betag

C'etag'

D'ETag'

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing to the string 'etag' instead of the variable etag.

Using 'ETag' which is not defined as a variable.

The variable etag holds the ETag string. We compare the header value to this variable without quotes.

4fill in blank

hard

Fill both blanks to set Cache-Control to no-store and disable ETag generation.

Express

app.use((req, res, next) => {
  res.set('[1]', 'no-store');
  app.disable('[2]');
  next();
});

Drag options to blanks, or click blank then click option'

ACache-Control

Betag

CETag

DCache

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ETag' instead of 'etag' in app.disable.

Setting wrong header names like 'Cache'.

Setting Cache-Control to no-store tells browsers not to cache. Disabling etag stops Express from generating ETags.

5fill in blank

hard

Fill both blanks to create a middleware that sets Cache-Control to private, sets ETag, and sends JSON response.

Express

app.get('/profile', (req, res) => {
  res.set('[1]', 'private, max-age=600');
  res.[2]();
  res.send({ user: 'Alice', id: 1, });
});

Drag options to blanks, or click blank then click option'

ACache-Control

Betag

C,

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Using semicolon instead of comma in JSON object.

Forgetting to call etag() method.

Setting wrong header name.

The middleware sets Cache-Control to private with max-age 600 seconds, calls etag() to generate the ETag header, and sends a JSON object with a comma separating properties.

Practice

1. What is the main purpose of the ETag header in Express HTTP responses?

easy

A. To redirect the client to a different URL

B. To specify how long the browser should cache the resource

C. To identify a specific version of a resource for caching validation

D. To indicate the content type of the response

Solution

1. Step 1: Understand the role of ETag

   The ETag header provides a unique identifier for a specific version of a resource, allowing the server to tell if the resource has changed.

2. Step 2: Compare with other headers

   Cache-Control sets caching duration, redirects use Location header, and content type is set by Content-Type header, so these are not related to ETag.

3. Final Answer:

   To identify a specific version of a resource for caching validation -> Option C

4. Quick Check:

   ETag = version ID for caching [OK]

Hint: ETag tags resource versions to avoid unnecessary downloads [OK]

Common Mistakes:

• Confusing ETag with Cache-Control duration
• Thinking ETag redirects clients
• Mixing ETag with content type headers

2. Which of the following is the correct way to set a Cache-Control header in Express to cache a response for 1 hour?

easy

A. res.set('Cache-Control', 'max-age=3600')

B. res.header('Cache-Control', 'no-cache')

C. res.cache('max-age=3600')

D. res.setHeader('Cache-Control', 'max-age=60')

Solution

1. Step 1: Identify Express method to set headers

   Express uses res.set() or res.header() to set headers. Both work, but res.set() is common and correct here.

2. Step 2: Check Cache-Control value for 1 hour

   1 hour = 3600 seconds, so max-age=3600 is correct. res.header('Cache-Control', 'no-cache') uses 'no-cache' which disables caching. res.cache('max-age=3600') is invalid method. res.setHeader('Cache-Control', 'max-age=60') sets max-age to 60 seconds, not 1 hour.

3. Final Answer:

   res.set('Cache-Control', 'max-age=3600') -> Option A

4. Quick Check:

   Cache-Control max-age=3600 sets 1 hour caching [OK]

Hint: Use res.set with 'max-age=seconds' for caching time [OK]

Common Mistakes:

• Using wrong method like res.cache
• Confusing seconds with minutes in max-age
• Setting no-cache when caching is needed

3. Given this Express code snippet, what will the client receive if the request includes header If-None-Match matching the ETag?

app.get('/data', (req, res) => {
  const data = { message: 'Hello' };
  const etag = '12345';
  res.set('ETag', etag);
  if (req.headers['if-none-match'] === etag) {
    res.status(304).end();
  } else {
    res.json(data);
  }
});

medium

A. A 500 Internal Server Error

B. A 304 Not Modified status with no body

C. A 404 Not Found error

D. A JSON response with { message: 'Hello' }

Solution

1. Step 1: Check If-None-Match header against ETag

   If the request's If-None-Match header equals the ETag '12345', the server responds with status 304 and no body.

2. Step 2: Understand 304 response meaning

   304 means Not Modified, telling the client to use cached data without sending the body again.

3. Final Answer:

   A 304 Not Modified status with no body -> Option B

4. Quick Check:

   If-None-Match matches ETag = 304 response [OK]

Hint: If ETag matches If-None-Match, send 304 no body [OK]

Common Mistakes:

• Sending JSON body with 304 status
• Confusing 304 with 404 or 500 errors
• Ignoring If-None-Match header

4. Identify the bug in this Express code that tries to set Cache-Control and ETag headers:

app.get('/resource', (req, res) => {
  const content = 'data';
  res.setHeader('Cache-Control', 'max-age=600');
  res.set('ETag', 'abc123');
  res.send(content);
});

medium

A. Using both res.setHeader and res.set causes header conflicts

B. ETag header value must be a number, not a string

C. Cache-Control max-age should be in milliseconds, not seconds

D. No bug; code sets headers correctly

Solution

1. Step 1: Check header setting methods

   In Express, res.setHeader and res.set both set headers correctly; using both is allowed and does not conflict.

2. Step 2: Validate header values

   ETag can be any string value. Cache-Control max-age is in seconds, so '600' is valid for 10 minutes.

3. Final Answer:

   No bug; code sets headers correctly -> Option D

4. Quick Check:

   res.setHeader and res.set both valid; ETag string OK [OK]

Hint: res.set and res.setHeader both valid for headers [OK]

Common Mistakes:

• Thinking ETag must be numeric
• Confusing max-age units (seconds vs milliseconds)
• Assuming res.setHeader and res.set conflict

5. You want to implement HTTP caching in Express so that clients cache a JSON response for 5 minutes, but also use ETag to validate changes. Which code snippet correctly combines Cache-Control and ETag headers to achieve this?

hard

A. res.set({ 'Cache-Control': 'max-age=300', ETag: 'xyz789' }); res.json(data);

B. res.set('Cache-Control', 'no-store'); res.set('ETag', 'xyz789'); res.json(data);

C. res.set('Cache-Control', 'max-age=300'); res.removeHeader('ETag'); res.json(data);

D. res.set('Cache-Control', 'max-age=300, no-cache'); res.set('ETag', 'xyz789'); res.json(data);

Solution

1. Step 1: Set Cache-Control for 5 minutes

   5 minutes = 300 seconds, so max-age=300 is correct to allow caching for that duration.

2. Step 2: Include ETag header for validation

   Setting ETag allows clients to check if the cached version is still valid before using it.

3. Step 3: Analyze other options

   res.set('Cache-Control', 'no-store'); res.set('ETag', 'xyz789'); res.json(data); disables caching with 'no-store'. res.set('Cache-Control', 'max-age=300'); res.removeHeader('ETag'); res.json(data); removes ETag, losing validation. res.set('Cache-Control', 'max-age=300, no-cache'); res.set('ETag', 'xyz789'); res.json(data); uses conflicting directives 'max-age=300, no-cache' which disables caching despite max-age.

4. Final Answer:

   res.set({ 'Cache-Control': 'max-age=300', ETag: 'xyz789' }); res.json(data); -> Option A

5. Quick Check:

   Cache-Control max-age + ETag = proper caching [OK]

Hint: Combine max-age with ETag for caching plus validation [OK]

Common Mistakes:

• Using no-store disables caching
• Removing ETag loses validation benefits
• Combining max-age with no-cache cancels caching