Cache middleware pattern in Express

Introduction

Cache middleware helps store data temporarily to make your app faster. It saves repeated work by remembering answers.

When your app fetches the same data many times quickly.

When you want to reduce slow database or API calls.

When you want to improve user experience by loading pages faster.

When you want to save server resources by avoiding repeated processing.

Syntax

Express

const cache = new Map();

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

The middleware checks if data for the request URL is in cache.

If cached, it sends the data immediately. Otherwise, it lets the request continue and saves the response.

Examples

Simple cache using a Map to store responses by URL.

Express

const cache = new Map();

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

Cache middleware with expiration time to clear old cache after given duration.

Express

const cache = new Map();

function cacheMiddleware(duration) {
  return (req, res, next) => {
    const key = req.originalUrl;
    if (cache.has(key)) {
      res.send(cache.get(key));
    } else {
      res.sendResponse = res.send;
      res.send = (body) => {
        cache.set(key, body);
        setTimeout(() => cache.delete(key), duration);
        res.sendResponse(body);
      };
      next();
    }
  };
}

Sample Program

This Express app uses cache middleware to save and reuse the response for the '/time' route. The first request shows the current time and caches it. Later requests return the cached time instantly.

Express

import express from 'express';

const app = express();
const cache = new Map();

function cacheMiddleware(req, res, next) {
  const key = req.originalUrl;
  if (cache.has(key)) {
    console.log('Serving from cache:', key);
    res.send(cache.get(key));
  } else {
    res.sendResponse = res.send;
    res.send = (body) => {
      cache.set(key, body);
      console.log('Caching response for:', key);
      res.sendResponse(body);
    };
    next();
  }
}

app.use(cacheMiddleware);

app.get('/time', (req, res) => {
  const currentTime = new Date().toISOString();
  res.send(`Current time is: ${currentTime}`);
});

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

Important Notes

Cache middleware improves speed but may serve outdated data if cache is not cleared.

Use cache expiration or manual clearing to keep data fresh.

Cache keys should be unique per request to avoid wrong data delivery.

Summary

Cache middleware stores responses to speed up repeated requests.

It checks cache before processing and saves new responses after.

Use cache expiration to avoid stale data.

Practice

1. What is the main purpose of cache middleware in an Express app?

easy

A. To handle user authentication

B. To store responses and speed up repeated requests

C. To parse incoming JSON data

D. To log all incoming requests

Solution

1. Step 1: Understand cache middleware role

   Cache middleware stores responses temporarily to avoid repeating expensive operations.

2. Step 2: Compare with other middleware roles

   Authentication, parsing, and logging serve different purposes unrelated to caching.

3. Final Answer:

   To store responses and speed up repeated requests -> Option B

4. Quick Check:

   Cache middleware = store responses [OK]

Hint: Cache middleware speeds up repeated requests by storing responses [OK]

Common Mistakes:

• Confusing cache with authentication middleware
• Thinking cache parses request bodies
• Assuming cache logs requests

2. Which of the following is the correct way to define a simple cache middleware function in Express?

easy

A. function cache(req, res, next) { next(); res.send(cacheData); }

B. function cache(req, res) { res.send(cacheData); next(); }

C. function cache(req, res, next) { if (cacheData) res.send(cacheData); else next(); }

D. function cache(req, next, res) { if (cacheData) res.send(cacheData); else next(); }

Solution

1. Step 1: Check middleware signature

   Express middleware functions take (req, res, next) in that order.

2. Step 2: Verify control flow

   If cached data exists, send it; otherwise, call next() to continue processing.

3. Final Answer:

   function cache(req, res, next) { if (cacheData) res.send(cacheData); else next(); } -> Option C

4. Quick Check:

   Middleware signature and flow = function cache(req, res, next) { if (cacheData) res.send(cacheData); else next(); } [OK]

Hint: Middleware must have (req, res, next) and call next() if no cache [OK]

Common Mistakes:

• Wrong parameter order in middleware function
• Calling next() after sending response
• Not calling next() when cache miss

3. Given this cache middleware snippet, what will be the output if cacheData is { message: 'Hello' }?

function cache(req, res, next) {
  if (cacheData) {
    res.json(cacheData);
  } else {
    next();
  }
}

app.get('/greet', cache, (req, res) => {
  res.json({ message: 'Hi there!' });
});

medium

A. {"message":"Hello"}

B. {"message":"Hi there!"}

C. Error: cacheData is undefined

D. No response sent

Solution

1. Step 1: Check cacheData value

   cacheData is defined as { message: 'Hello' }, so the if condition is true.

2. Step 2: Determine middleware behavior

   Since cacheData exists, middleware sends JSON response and does not call next().

3. Final Answer:

   {"message":"Hello"} -> Option A

4. Quick Check:

   cacheData present sends cached response [OK]

Hint: If cacheData exists, middleware sends it and skips next() [OK]

Common Mistakes:

• Assuming next() runs even if cacheData exists
• Confusing JSON string with object
• Expecting original handler response

4. Identify the bug in this cache middleware code:

let cacheData;

function cache(req, res, next) {
  if (cacheData) {
    res.send(cacheData);
  }
  next();
}

medium

A. Middleware function missing req parameter

B. cacheData is not initialized

C. res.send should be res.json

D. next() is called even after sending a response

Solution

1. Step 1: Analyze control flow

   If cacheData exists, res.send(cacheData) sends a response, but next() is still called afterward.

2. Step 2: Understand Express behavior

   Calling next() after sending a response can cause errors or duplicate responses.

3. Final Answer:

   next() is called even after sending a response -> Option D

4. Quick Check:

   Calling next() after res.send causes errors [OK]

Hint: Do not call next() after sending a response [OK]

Common Mistakes:

• Calling next() unconditionally after res.send
• Ignoring middleware flow control
• Assuming res.send and next() can run together

5. You want to implement cache middleware that stores responses for 10 seconds and then expires. Which approach correctly achieves this in Express?

hard

A. Store response in cacheData with a timestamp; clear cacheData if older than 10 seconds before sending

B. Use setTimeout to clear cacheData every 10 seconds regardless of requests

C. Send cached response only if cacheData exists, without checking time

D. Clear cacheData manually by restarting the server every 10 seconds

Solution

1. Step 1: Understand cache expiration logic

   Cache should store data with a timestamp and check age before sending cached response.

2. Step 2: Evaluate options

   Store response in cacheData with a timestamp; clear cacheData if older than 10 seconds before sending checks timestamp and clears stale cache before responding, ensuring fresh data.

3. Final Answer:

   Store response in cacheData with a timestamp; clear cacheData if older than 10 seconds before sending -> Option A

4. Quick Check:

   Cache expiration needs timestamp check [OK]

Hint: Use timestamps to expire cache, not just timers or manual clears [OK]

Common Mistakes:

• Ignoring cache expiration timing
• Clearing cache blindly without request context
• Relying on server restart to clear cache