ETag and conditional requests in Node.js - Performance & Optimization

Performance: ETag and conditional requests

MEDIUM IMPACT

This concept affects page load speed by reducing unnecessary data transfer and server processing for unchanged resources.

Serving static files efficiently with caching

Node.js

app.get('/file', (req, res) => {
  const fileContent = fs.readFileSync('file.txt');
  const etag = generateETag(fileContent);
  if (req.headers['if-none-match'] === etag) {
    res.status(304).end();
  } else {
    res.setHeader('ETag', etag);
    res.setHeader('Content-Type', 'text/plain');
    res.send(fileContent);
  }
});

function generateETag(content) {
  return 'W/"' + require('crypto').createHash('md5').update(content).digest('hex') + '"';
}

Sends 304 Not Modified when content is unchanged, avoiding full data transfer and speeding up load.

📈 Performance GainReduces bandwidth usage and improves LCP by skipping full resource download on cache hits.

Serving static files efficiently with caching

Node.js

app.get('/file', (req, res) => {
  const fileContent = fs.readFileSync('file.txt');
  res.setHeader('Content-Type', 'text/plain');
  res.send(fileContent);
});

Always sends full file content on every request, causing unnecessary data transfer and slower load times.

📉 Performance CostBlocks rendering until full file is downloaded every time, increasing LCP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
No ETag, always full response	N/A	N/A	High network and paint cost due to full reload	[X] Bad
ETag with conditional requests	N/A	N/A	Low network cost, faster paint due to cached content	[OK] Good

Rendering Pipeline

ETag and conditional requests reduce the amount of data the browser must download and parse, speeding up the critical rendering path by avoiding unnecessary resource loading.

→Network

→Style Calculation

→Layout

→Paint

⚠️ BottleneckNetwork transfer and resource loading

Core Web Vital Affected

LCP

This concept affects page load speed by reducing unnecessary data transfer and server processing for unchanged resources.

Optimization Tips

1Always generate and send ETag headers for cacheable resources.

2Check the If-None-Match header to respond with 304 when content is unchanged.

3Avoid sending full resource data when the client already has the latest version.

Performance Quiz - 3 Questions

What is the main performance benefit of using ETag with conditional requests?

A Improves CSS selector matching speed

B Increases server CPU usage by recalculating content

C Reduces data transfer by avoiding sending unchanged resources

D Reduces JavaScript execution time

DevTools: Network

How to check: Open DevTools, go to Network tab, reload the page, and look for 304 status codes in the resource list.

What to look for: Resources returning 304 Not Modified indicate successful conditional requests and efficient caching.

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