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.