How do you implement a custom caching middleware for Express?

Question

Accepted Answer

**Cache middleware** — cache responses for performance.

```javascript
function cacheMiddleware(options = {}) {
    const {
        ttl = 60,
        keyGenerator = (req) => req.originalUrl,
        condition = () => true,
    } = options;

const cache = new Map();

return (req, res, next) => {
        if (!condition(req)) return next();

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

if (cached && Date.now() - cached.timestamp < ttl * 1000) {
            res.setHeader("X-Cache", "HIT");
            return res.json(cached.data);
        }

res.setHeader("X-Cache", "MISS");
        const originalJson = res.json.bind(res);

res.json = (data) => {
            cache.set(key, {
                data,
                timestamp: Date.now(),
            });
            return originalJson(data);
        };

next();
    };
}

// Usage
app.get("/products",
    cacheMiddleware({
        ttl: 300,
        keyGenerator: (req) => `products:${req.query.page}`,
        condition: (req) => req.method === "GET",
    }),
    async (req, res) => {
        const products = await db.getProducts(req.query);
        res.json(products);
    }
);

// Invalidate cache
app.post("/products", async (req, res) => {
    const product = await db.createProduct(req.body);
    cache.delete("products:1"); // invalidate
    res.json(product);
});
```

Node.js: How do you implement a custom caching middleware for Express?