ETag and conditional requests in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of an ETag in HTTP?

ETags are used in HTTP to help with which of the following?

A To uniquely identify a specific version of a resource for caching and conditional requests

B To specify the language preference of the client

C To encrypt the content of the HTTP response for security

D To indicate the server's processing time for the request

💡 Hint

Think about how browsers avoid downloading unchanged files.

An ETag is a unique identifier assigned to a specific version of a resource. It helps clients and servers determine if the resource has changed, enabling efficient caching and conditional requests.

❓ component_behavior

intermediate

What happens when a client sends an If-None-Match header with a matching ETag?

In a Node.js server using ETags, if the client sends an If-None-Match header with an ETag that matches the current resource's ETag, what should the server respond with?

A Respond with status 500 Internal Server Error

B Respond with status 200 OK and the full resource body

C Respond with status 404 Not Found

D Respond with status 304 Not Modified and no response body

💡 Hint

Consider what happens when the resource has not changed.

If the ETag matches, the server knows the client has the latest version and responds with 304 Not Modified, saving bandwidth by not sending the body again.

📝 Syntax

advanced

Which Node.js code snippet correctly sets an ETag header and handles conditional requests?

Given a resource string, which code snippet correctly sets the ETag header and returns 304 if the client's If-None-Match matches the ETag?

Node.js

const http = require('http');
const resource = 'Hello World';
const etag = '"12345"'; // example ETag

http.createServer((req, res) => {
  // Your code here
}).listen(3000);

A

res.setHeader('ETag', etag);
if (req.headers['if-none-match'] !== etag) {
  res.statusCode = 304;
  res.end();
} else {
  res.statusCode = 200;
  res.end(resource);
}

B

res.setHeader('ETag', etag);
if (req.headers['if-none-match'] === etag) {
  res.statusCode = 304;
  res.end();
} else {
  res.statusCode = 200;
  res.end(resource);
}

C

res.setHeader('ETag', etag);
if (req.headers['if-none-match'] === etag) {
  res.statusCode = 200;
  res.end(resource);
} else {
  res.statusCode = 304;
  res.end();
}

D

res.setHeader('ETag', etag);
res.statusCode = 200;
res.end(resource);

💡 Hint

Check the condition for matching ETag and the correct status code.

Option B correctly sets the ETag header, checks if the client's If-None-Match matches the ETag, and responds with 304 if it does, otherwise sends the resource with 200.

🔧 Debug

advanced

Why does this Node.js server always send the full resource even if the ETag matches?

Consider this code snippet: const http = require('http'); const resource = 'Hello World'; const etag = '"12345"'; http.createServer((req, res) => { res.setHeader('ETag', etag); if (req.headers['if-none-match'] == etag) { res.statusCode = 304; } res.statusCode = 200; res.end(resource); }).listen(3000); Why does the server always send the full resource body even when the ETag matches?

A Because the statusCode is overwritten to 200 after setting it to 304, so 304 is never sent

B Because the ETag header is not set correctly

C Because the if condition uses == instead of === causing a type mismatch

D Because the server does not check the If-Modified-Since header

💡 Hint

Look at the order of setting statusCode and ending the response.

The code sets statusCode to 304 inside the if block but then immediately overwrites it to 200 before sending the response, so the client always receives 200 with the full body.

❓ state_output

expert

What is the response status and body when the client sends a non-matching If-None-Match header?

Given this Node.js server snippet: const http = require('http'); const resource = 'Data v1'; const etag = '"v1"'; http.createServer((req, res) => { res.setHeader('ETag', etag); if (req.headers['if-none-match'] === etag) { res.statusCode = 304; res.end(); } else { res.statusCode = 200; res.end(resource); } }).listen(3000); If the client sends an If-None-Match header with value "v2", what will be the response status and body?

A Status 304 with no body

B Status 404 with no body

C Status 200 with body 'Data v1'

D Status 500 with error message

💡 Hint

If the ETag does not match, the server sends the full resource.

Since the If-None-Match header value "v2" does not match the current ETag "v1", the server responds with 200 OK and sends the resource body.

Practice

1. What is the main purpose of using an ETag in HTTP responses in Node.js?

easy

A. To set the cache expiration time

B. To encrypt the response data for security

C. To specify the content type of the response

D. To help browsers check if the content has changed and avoid re-downloading unchanged data

Solution

1. Step 1: Understand what ETag represents

   ETag is a unique identifier for a specific version of a resource, used to detect changes.

2. Step 2: Identify its role in HTTP caching

   Browsers use ETag to check if the resource changed since last download, avoiding unnecessary data transfer.

3. Final Answer:

   To help browsers check if the content has changed and avoid re-downloading unchanged data -> Option D

4. Quick Check:

   ETag purpose = avoid re-download [OK]

Hint: ETag means "same content? skip download" [OK]

Common Mistakes:

• Confusing ETag with encryption
• Thinking ETag sets content type
• Mixing ETag with cache expiration headers

2. Which of the following is the correct way to set an ETag header in a Node.js HTTP response?

easy

A. res.setHeader('ETag', '12345')

B. res.header('etag', 12345)

C. res.etag = '12345'

D. res.set('ETag', 12345)

Solution

1. Step 1: Recall Node.js HTTP response header syntax

   Use res.setHeader(name, value) to set headers correctly.

2. Step 2: Check the correct header name and value type

   Header name is case-insensitive but usually 'ETag', and value must be a string.

3. Final Answer:

   res.setHeader('ETag', '12345') -> Option A

4. Quick Check:

   Set header with res.setHeader('ETag', value) [OK]

Hint: Use res.setHeader('ETag', 'value') to set ETag [OK]

Common Mistakes:

• Using incorrect method names like res.header or res.set
• Passing number instead of string as header value
• Trying to assign ETag directly as a property

3. Consider this Node.js code snippet handling a request with an ETag check:

const etag = 'abc123';
if (req.headers['if-none-match'] === etag) {
  res.statusCode = 304;
  res.end();
} else {
  res.setHeader('ETag', etag);
  res.end('Hello World');
}

What will the server respond if the client sends header If-None-Match: abc123?

medium

A. Respond with status 200 and body 'Hello World'

B. Respond with status 500 Internal Server Error

C. Respond with status 304 and no body

D. Respond with status 404 Not Found

Solution

1. Step 1: Check the condition comparing If-None-Match header

   The code compares req.headers['if-none-match'] to etag which is 'abc123'. If equal, it sets status 304.

2. Step 2: Understand HTTP 304 meaning

   Status 304 means Not Modified, so no body is sent and client uses cached content.

3. Final Answer:

   Respond with status 304 and no body -> Option C

4. Quick Check:

   If-None-Match matches ETag = 304 no body [OK]

Hint: If-None-Match equals ETag means 304 Not Modified [OK]

Common Mistakes:

• Thinking 304 responses include body content
• Confusing 304 with 200 OK
• Ignoring the If-None-Match header check

4. This Node.js code tries to handle ETag conditional requests but has a bug:

const etag = 'xyz789';
if (req.headers['If-None-Match'] === etag) {
  res.statusCode = 304;
  res.end();
} else {
  res.setHeader('etag', etag);
  res.end('Content');
}

What is the bug causing the ETag check to fail?

medium

A. ETag value should be a number, not a string

B. Header name case mismatch: 'If-None-Match' vs 'if-none-match'

C. res.end() is missing a callback function

D. res.statusCode must be set after res.end()

Solution

1. Step 1: Check header name case sensitivity in Node.js

   Node.js lowercases all incoming header names, so req.headers['If-None-Match'] is undefined; correct is req.headers['if-none-match'].

2. Step 2: Confirm impact on ETag comparison

   Because the header is undefined, the condition never matches, so 304 is never sent.

3. Final Answer:

   Header name case mismatch: 'If-None-Match' vs 'if-none-match' -> Option B

4. Quick Check:

   Headers lowercase in Node.js = use 'if-none-match' [OK]

Hint: Node.js headers are lowercase keys [OK]

Common Mistakes:

• Using wrong case for header keys
• Thinking ETag must be numeric
• Misordering res.statusCode and res.end()

5. You want to optimize your Node.js server to use ETags and respond with 304 when content is unchanged. Which approach correctly combines generating an ETag and handling conditional requests?

hard

A. Generate a hash of the response body, set it as ETag header, then compare with req.headers['if-none-match'] to decide 304 or 200

B. Always send status 200 with ETag header, ignoring If-None-Match header

C. Set ETag header only on POST requests and ignore GET requests

D. Use res.setHeader('Cache-Control', 'no-store') to disable caching instead of ETags

Solution

1. Step 1: Understand ETag generation

   ETags should uniquely represent the current content, often by hashing the response body.

2. Step 2: Use conditional request header to decide response

   Compare req.headers['if-none-match'] with generated ETag; if equal, respond 304, else send content with new ETag.

3. Final Answer:

   Generate a hash of the response body, set it as ETag header, then compare with req.headers['if-none-match'] to decide 304 or 200 -> Option A

4. Quick Check:

   Hash body -> set ETag -> compare If-None-Match -> 304 or 200 [OK]

Hint: Hash content, set ETag, compare If-None-Match for 304 [OK]

Common Mistakes:

• Ignoring If-None-Match header
• Sending 200 always without checking ETag
• Misusing Cache-Control to replace ETag logic