Response caching middleware — how to cache API responses

API Response Caching Middleware

Caching stores the result of expensive operations so repeated requests get the cached response instead of re-running the work.

Simple in-memory cache middleware

const cache = new Map();

function cacheMiddleware(ttlSeconds = 60) {
  return (req, res, next) => {
    if (req.method !== 'GET') return next(); // only cache GET

    const key = req.originalUrl;
    const cached = cache.get(key);

    if (cached && Date.now() < cached.expiresAt) {
      res.set('X-Cache', 'HIT');
      return res.json(cached.data);
    }

    // Intercept res.json to capture the response
    const originalJson = res.json.bind(res);
    res.json = (data) => {
      cache.set(key, { data, expiresAt: Date.now() + ttlSeconds * 1000 });
      res.set('X-Cache', 'MISS');
      return originalJson(data);
    };

    next();
  };
}

// Cache public endpoints for 5 minutes
app.get('/products', cacheMiddleware(300), getProducts);
app.get('/categories', cacheMiddleware(600), getCategories);

The X-Cache: HIT/MISS header lets you verify caching in browser DevTools. In-memory caching doesn't survive restarts — use Redis for production. Never cache authenticated endpoints or responses containing user-specific data.