Conditional requests handling in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a client sends an If-None-Match header matching the ETag?

Consider this Express route that serves a resource with an ETag header:

app.get('/resource', (req, res) => {
  const data = 'Hello World';
  const etag = '12345';
  res.set('ETag', etag);
  if (req.headers['if-none-match'] === etag) {
    res.status(304).end();
  } else {
    res.send(data);
  }
});

If a client sends a request with header If-None-Match: 12345, what will the server respond with?

Express

app.get('/resource', (req, res) => {
  const data = 'Hello World';
  const etag = '12345';
  res.set('ETag', etag);
  if (req.headers['if-none-match'] === etag) {
    res.status(304).end();
  } else {
    res.send(data);
  }
});

A Status 500 Internal Server Error

B Status 304 with no body

C Status 404 Not Found

D Status 200 with body 'Hello World'

💡 Hint

Think about what the 304 status code means in HTTP.

If the client's If-None-Match header matches the server's ETag, the server responds with 304 Not Modified and no body, telling the client to use its cached version.

📝 Syntax

intermediate

Which option correctly sets Last-Modified header and handles If-Modified-Since?

Given this Express route, which option correctly implements conditional GET using Last-Modified?

app.get('/file', (req, res) => {
  const lastModified = new Date('2024-01-01T12:00:00Z');
  // Your code here
});

Express

app.get('/file', (req, res) => {
  const lastModified = new Date('2024-01-01T12:00:00Z');
  // Your code here
});

A

res.set('Last-Modified', lastModified.toUTCString());
if (req.headers['if-modified-since'] > lastModified.toUTCString()) {
  res.status(304).end();
} else {
  res.send('File content');
}

B

res.set('Last-Modified', lastModified);
if (req.headers['if-modified-since'] === lastModified.toUTCString()) {
  res.status(304).end();
} else {
  res.send('File content');
}

C

res.set('Last-Modified', lastModified.toUTCString());
if (new Date(req.headers['if-modified-since']) >= lastModified) {
  res.status(304).end();
} else {
  res.send('File content');
}

D

res.set('Last-Modified', lastModified.toISOString());
if (new Date(req.headers['if-modified-since']) < lastModified) {
  res.status(304).end();
} else {
  res.send('File content');
}

💡 Hint

Remember to compare dates as Date objects, not strings.

Option C correctly sets the Last-Modified header as a UTC string and compares the client's If-Modified-Since date as a Date object to decide if the resource is modified.

🔧 Debug

advanced

Why does this Express code always send the full response ignoring If-None-Match?

Examine this code snippet:

app.get('/data', (req, res) => {
  const etag = 'abc123';
  res.set('ETag', etag);
  if (req.headers['if-none-match'] !== etag) {
    res.send('Data content');
  } else {
    res.status(304).end();
  }
});

Clients report they always get the full content, even when sending If-None-Match: abc123. What is the bug?

Express

app.get('/data', (req, res) => {
  const etag = 'abc123';
  res.set('ETag', etag);
  if (req.headers['if-none-match'] !== etag) {
    res.send('Data content');
  } else {
    res.status(304).end();
  }
});

A The If-None-Match header may contain quotes, so direct string comparison fails

B ETag header must be set after sending the response

C The comparison should use strict equality === instead of !==

D The status 304 must be sent before setting the ETag header

💡 Hint

Check the exact value format of the If-None-Match header sent by browsers.

Browsers send ETag values with quotes, e.g., "abc123". The code compares without quotes, so the condition is always true, sending full content.

❓ state_output

advanced

What is the response status and body when If-Modified-Since is before Last-Modified?

Given this Express route:

app.get('/info', (req, res) => {
  const lastModified = new Date('2024-06-01T10:00:00Z');
  res.set('Last-Modified', lastModified.toUTCString());
  const ifModifiedSince = new Date(req.headers['if-modified-since']);
  if (ifModifiedSince >= lastModified) {
    res.status(304).end();
  } else {
    res.send('Updated info');
  }
});

If the client sends If-Modified-Since: 2024-05-01T10:00:00Z, what will the server respond?

Express

app.get('/info', (req, res) => {
  const lastModified = new Date('2024-06-01T10:00:00Z');
  res.set('Last-Modified', lastModified.toUTCString());
  const ifModifiedSince = new Date(req.headers['if-modified-since']);
  if (ifModifiedSince >= lastModified) {
    res.status(304).end();
  } else {
    res.send('Updated info');
  }
});

A Status 200 with body 'Updated info'

B Status 304 with no body

C Status 400 Bad Request

D Status 500 Internal Server Error

💡 Hint

Compare the dates carefully to see if the resource is newer than the client's cached version.

The client's If-Modified-Since date is before the resource's Last-Modified date, so the resource has changed. The server sends status 200 with the updated content.

🧠 Conceptual

expert

Which HTTP header combination best supports efficient conditional GET caching?

To implement efficient conditional GET caching in an Express app, which combination of headers should be used together?

A ETag and Cache-Control headers

B Last-Modified and If-Modified-Since headers

C Cache-Control and Pragma headers

D ETag and If-None-Match headers

💡 Hint

Think about which headers allow precise validation of resource changes.

ETag and If-None-Match headers provide a strong validator for resource changes, enabling precise conditional GET requests. Last-Modified is less precise and can be used alone but ETag is preferred for accuracy.