Conditional requests handling in Express - Performance & Optimization

Performance: Conditional requests handling

MEDIUM IMPACT

This affects server response time and network load by reducing unnecessary data transfer when content is unchanged.

Serving static or dynamic content efficiently with caching

Express

app.get('/data', (req, res) => {
  const data = getData();
  const etag = generateETag(data);
  res.set('ETag', etag);
  if (req.headers['if-none-match'] === etag) {
    res.status(304).end();
  } else {
    res.send(data);
  }
});

Sends 304 Not Modified when content is unchanged, saving bandwidth and speeding up response.

📈 Performance GainReduces payload size and improves LCP by avoiding full data transfer on cached requests.

Serving static or dynamic content efficiently with caching

Express

app.get('/data', (req, res) => {
  const data = getData();
  res.send(data);
});

Always sends full response regardless of client cache, causing unnecessary data transfer and slower load times.

📉 Performance CostIncreases network payload and delays LCP due to full content transfer every request.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Always send full response	N/A (server-side)	N/A	High network payload delays paint	[X] Bad
Use ETag with 304 Not Modified	N/A (server-side)	N/A	Minimal network payload speeds paint	[OK] Good

Rendering Pipeline

Conditional requests reduce server response size, which speeds up the network transfer stage and allows the browser to render content faster.

→Network Transfer

→First Contentful Paint

→Largest Contentful Paint

⚠️ BottleneckNetwork Transfer due to large payloads

Core Web Vital Affected

LCP

This affects server response time and network load by reducing unnecessary data transfer when content is unchanged.

Optimization Tips

1Always set ETag or Last-Modified headers for cacheable responses.

2Respond with 304 Not Modified when client cache is valid.

3Avoid sending full response if content has not changed to save bandwidth and improve load speed.

Performance Quiz - 3 Questions

What is the main benefit of handling conditional requests with ETag in Express?

A Increases server CPU usage

B Forces full page reload every time

C Reduces data sent when content is unchanged

D Disables browser caching

DevTools: Network

How to check: Open DevTools, go to Network tab, reload page, and inspect the response status code and size for your resource.

What to look for: Look for 304 status codes and small response sizes indicating conditional requests worked.