Cache middleware pattern in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this cache middleware do when a cached response exists?

Consider this Express cache middleware snippet. What happens when a cached response is found?

Express

const cache = {};

function cacheMiddleware(req, res, next) {
  const key = req.originalUrl;
  if (cache[key]) {
    res.send(cache[key]);
  } else {
    res.sendResponse = res.send;
    res.send = (body) => {
      cache[key] = body;
      res.sendResponse(body);
    };
    next();
  }
}

A It throws an error because res.send is overwritten incorrectly.

B It sends the cached response immediately without calling next().

C It clears the cache and then calls next().

D It calls next() and then sends the cached response.

💡 Hint

Think about what happens when cache[key] exists and how res.send is used.

If a cached response exists, the middleware sends it immediately using res.send and does not call next(), so the request does not proceed further.

📝 Syntax

intermediate

Which option correctly caches the response body in Express middleware?

Choose the correct way to cache the response body inside Express middleware.

Express

const cache = {};

function cacheMiddleware(req, res, next) {
  const key = req.originalUrl;
  if (cache[key]) {
    res.send(cache[key]);
  } else {
    res.sendResponse = res.send;
    res.send = (body) => {
      // Cache the body here
      res.sendResponse(body);
    };
    next();
  }
}

A cache[key] = body;

B cache[key] == body;

C cache[key] === body;

D cache.key = body;

💡 Hint

Remember how to assign a value to an object property using a variable key.

To store the response body in the cache object using the key variable, use cache[key] = body; This assigns the value correctly.

🔧 Debug

advanced

Why does this cache middleware cause the response to never send?

Identify the bug in this cache middleware that causes the client to hang without receiving a response.

Express

const cache = {};

function cacheMiddleware(req, res, next) {
  const key = req.originalUrl;
  if (cache[key]) {
    res.send(cache[key]);
  } else {
    res.send = (body) => {
      cache[key] = body;
      res.send(body);
    };
    next();
  }
}

A res.send is overwritten and calls itself recursively causing a stack overflow.

B cache[key] is never assigned so cache is always empty.

C next() is not called so the request hangs.

D res.sendResponse is missing causing a syntax error.

💡 Hint

Look at how res.send is redefined and what happens when it calls res.send inside itself.

Overwriting res.send and then calling res.send inside the new function causes infinite recursion. This leads to a stack overflow and the response never sends.

❓ state_output

advanced

What is the cache content after these requests?

Given this cache middleware and two requests to '/data', what is the cache content after both requests?

Express

const cache = {};

function cacheMiddleware(req, res, next) {
  const key = req.originalUrl;
  if (cache[key]) {
    res.send(cache[key]);
  } else {
    res.sendResponse = res.send;
    res.send = (body) => {
      cache[key] = body;
      res.sendResponse(body);
    };
    next();
  }
}

// Simulated requests:
// 1st request to '/data' sends 'First response'
// 2nd request to '/data' sends 'Second response'

// Assume middleware is used correctly and responses are sent accordingly.

A {}

B {"/data": "Second response"}

C {"/data": "First response"}

D {"/data": null}

💡 Hint

Think about when the cache is updated and when the cached response is sent.

The first request caches 'First response'. The second request finds the cache and sends the cached 'First response' without updating it. So cache remains with the first response.

🧠 Conceptual

expert

What is a key limitation of this simple cache middleware pattern?

Consider this cache middleware pattern that stores responses in memory keyed by URL. What is a key limitation of this approach in a real-world Express app?

Express

const cache = {};

function cacheMiddleware(req, res, next) {
  const key = req.originalUrl;
  if (cache[key]) {
    res.send(cache[key]);
  } else {
    res.sendResponse = res.send;
    res.send = (body) => {
      cache[key] = body;
      res.sendResponse(body);
    };
    next();
  }
}

A It encrypts cached data to protect user privacy.

B It automatically scales across multiple server instances without extra configuration.

C It supports caching POST requests with different bodies automatically.

D It does not handle cache invalidation or expiration, so stale data may be served indefinitely.

💡 Hint

Think about what happens if the data changes but the cache never updates or clears.

This simple cache stores data indefinitely in memory without any expiration or invalidation logic. This can cause the app to serve outdated responses if the underlying data changes.