Conditional requests handling in Express

Introduction

Conditional requests help your server save work by sending data only if it has changed. This makes websites faster and uses less internet data.

When you want to avoid sending the same data again if the client already has the latest version.

When building APIs that return resources which might not change often.

When you want to improve performance by reducing unnecessary data transfer.

When handling browser caching to make pages load faster.

When you want to support HTTP caching headers like ETag or Last-Modified.

Syntax

Express

app.get('/resource', (req, res) => {
  res.set('ETag', 'abc123');
  if (req.fresh) {
    res.status(304).end();
  } else {
    res.send('Your data here');
  }
});

req.fresh is true if the client's cached version is up to date.

Sending status 304 Not Modified tells the client to use its cached data.

Examples

This example uses an ETag header to help the client check if data changed.

Express

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

This example uses Last-Modified header to tell when data was last changed.

Express

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

Sample Program

This server sends a message with an ETag header. If the client sends the same ETag back, the server replies with 304 Not Modified, saving bandwidth.

Express

import express from 'express';
const app = express();

app.get('/message', (req, res) => {
  const message = 'Hello from server!';
  const etag = 'v1';

  res.set('ETag', etag);

  if (req.headers['if-none-match'] === etag) {
    res.status(304).end();
  } else {
    res.send(message);
  }
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Always set ETag or Last-Modified headers to enable conditional requests.

Use req.fresh or check if-none-match and if-modified-since headers to decide if data changed.

Responding with 304 means no body is sent, so the client uses cached data.

Summary

Conditional requests help reduce data sent by checking if client cache is fresh.

Use ETag or Last-Modified headers to support conditional requests.

Respond with 304 status to tell client to use cached data.

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