Conditional requests handling in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of conditional requests in Express?

Conditional requests help the server decide if it needs to send the full response or just a status like 304 Not Modified, saving bandwidth and improving speed.

beginner

Which HTTP headers are commonly used for conditional requests?

The main headers are If-None-Match (for ETag) and If-Modified-Since (for last modified date). They tell the server what version the client has.

intermediate

How does Express help handle conditional GET requests?

Express provides the res.sendFile() and res.send() methods that automatically check request headers and send a 304 status if the resource is unchanged.

intermediate

What is an ETag and how is it used in Express conditional requests?

An ETag is a unique identifier for a resource version. Express can set ETags automatically, and clients send them back in If-None-Match headers to check if the resource changed.

advanced

How can you manually handle conditional requests in Express?

You can check request headers like req.headers['if-none-match'] or req.headers['if-modified-since'], compare with your resource state, and respond with res.status(304).end() if unchanged.

Which status code indicates the resource has NOT changed in a conditional request?

A 200 OK

B 500 Internal Server Error

C 404 Not Found

D 304 Not Modified

304 Not Modified tells the client the cached resource is still valid, so no new data is sent.

Which header does the client send to check if the resource's ETag matches?

A If-None-Match

B If-Modified-Since

C ETag

D Content-Type

If-None-Match carries the ETag value from the client to check if the resource changed.

What does Express do when you use res.sendFile() with conditional headers?

A Always sends full file content

B Automatically sends 304 if resource is unchanged

C Ignores conditional headers

D Throws an error

Express checks conditional headers and sends 304 Not Modified if the file hasn't changed.

Which header is used to check if a resource has been modified since a certain date?

A Last-Modified

B If-None-Match

C If-Modified-Since

D Content-Length

If-Modified-Since lets the client ask if the resource changed after a specific date.

How can you respond with 304 Not Modified manually in Express?

A res.status(304).end()

B res.send(304)

C res.json(304)

D res.redirect(304)

Use res.status(304).end() to send a 304 status with no body.

Explain how Express handles conditional GET requests using ETag and If-None-Match headers.

Think about how the server knows if the client has the latest version.

Describe how you would manually implement conditional request handling in an Express route.

Focus on checking headers and deciding the response status.

Practice

1. What is the main purpose of handling conditional requests in Express?

easy

A. To increase server processing time

B. To always send full response data regardless of cache

C. To reduce data sent by checking if the client cache is fresh

D. To disable client caching completely

Solution

1. Step 1: Understand conditional requests concept

   Conditional requests check if the client already has fresh data to avoid sending it again.

2. Step 2: Identify the purpose in Express

   Express uses conditional requests to reduce unnecessary data transfer by responding with 304 if data is fresh.

3. Final Answer:

   To reduce data sent by checking if the client cache is fresh -> Option C

4. Quick Check:

   Conditional requests reduce data transfer = B [OK]

Hint: Conditional requests save bandwidth by checking cache freshness [OK]

Common Mistakes:

• Thinking conditional requests increase server load
• Assuming full data is always sent
• Believing caching is disabled by conditional requests

2. Which Express property is used to check if the request is fresh based on ETag or Last-Modified headers?

easy

A. res.isFresh()

B. res.checkFresh()

C. req.isFresh()

D. req.fresh

Solution

1. Step 1: Recall Express request object properties

   The freshness of a request is checked on the request object, not the response.

2. Step 2: Identify the correct property name

   Express provides req.fresh to check if the request is fresh based on headers.

3. Final Answer:

   req.fresh -> Option D

4. Quick Check:

   Check freshness with req.fresh = B [OK]

Hint: Use req.fresh to check cache freshness in Express [OK]

Common Mistakes:

• Using res.isFresh() instead of req.fresh
• Confusing property names like req.isFresh() which don't exist
• Trying to check freshness on response object

3. What will the following Express code send if the client sends a matching ETag header?

app.get('/data', (req, res) => {
  res.set('ETag', '12345');
  if (req.fresh) {
    res.status(304).end();
  } else {
    res.send('Full data response');
  }
});

medium

A. Full data response with status 200

B. Status 304 with no body

C. Status 500 error

D. Empty response with status 200

Solution

1. Step 1: Understand ETag and req.fresh

   The server sets ETag '12345'. If the client sends the same ETag, req.fresh returns true.

2. Step 2: Analyze the conditional response

   If req.fresh is true, the server responds with status 304 and ends response without body.

3. Final Answer:

   Status 304 with no body -> Option B

4. Quick Check:

   Matching ETag triggers 304 no body = A [OK]

Hint: Matching ETag means send 304 status with empty body [OK]

Common Mistakes:

• Sending full data even when ETag matches
• Returning status 200 with empty body
• Confusing 304 with error status

4. Identify the error in this Express route handling conditional requests:

app.get('/info', (req, res) => {
  res.set('Last-Modified', new Date().toUTCString());
  if (req.fresh()) {
    res.status(304).end();
  } else {
    res.send('Info data');
  }
});

medium

A. Calling req.fresh() with parentheses (it's a property)

B. Setting Last-Modified header incorrectly

C. Missing res.send() in both branches

D. Using arrow function incorrectly

Solution

1. Step 1: Check usage of req.fresh

   req.fresh is a property and must be accessed without parentheses: req.fresh

2. Step 2: Verify other parts

   Last-Modified header is set correctly, res.send() is called in else branch, arrow function syntax is correct.

3. Final Answer:

   Calling req.fresh() with parentheses (it's a property) -> Option A

4. Quick Check:

   Property access needs no () = A [OK]

Hint: Always access req.fresh without parentheses [OK]

Common Mistakes:

• Treating req.fresh as a method by adding parentheses
• Misformatting Last-Modified header
• Not sending response in all branches

5. You want to optimize an Express API to support conditional requests using both ETag and Last-Modified headers. Which approach correctly implements this to respond with 304 when appropriate? A) Set ETag and Last-Modified headers, then check req.fresh to send 304 or full data. B) Only set ETag header and always send full data. C) Set Last-Modified header but never check req.fresh, always send 200. D) Use req.fresh without setting any headers.

hard

A. Set ETag and Last-Modified headers, then check req.fresh to send 304 or full data

B. Only set ETag header and always send full data

C. Set Last-Modified header but never check req.fresh, always send 200

D. Use req.fresh without setting any headers

Solution

1. Step 1: Understand headers and freshness check

   ETag and Last-Modified headers must be set for conditional requests to work properly.

2. Step 2: Use req.fresh to decide response

   req.fresh checks these headers and client cache; if fresh, respond 304, else send full data.

3. Step 3: Evaluate options

   Only Set ETag and Last-Modified headers, then check req.fresh to send 304 or full data: Set ETag and Last-Modified headers, then check req.fresh to send 304 or full data correctly sets headers and uses req.fresh to conditionally send 304 or data.

4. Final Answer:

   Set ETag and Last-Modified headers, then check req.fresh to send 304 or full data -> Option A

5. Quick Check:

   Set headers + check req.fresh = D [OK]

Hint: Set headers and check req.fresh to send 304 or data [OK]

Common Mistakes:

• Not setting headers but checking freshness
• Setting headers but ignoring req.fresh
• Using req.fresh without headers