How do you implement a custom request/response interceptor?

Question

Accepted Answer

**Interceptor** — modify requests/responses before they reach handlers.

```javascript
class Interceptor {
    constructor() {
        this.requestInterceptors = [];
        this.responseInterceptors = [];
    }

addRequestInterceptor(fn) {
        this.requestInterceptors.push(fn);
    }

addResponseInterceptor(fn) {
        this.responseInterceptors.push(fn);
    }

async processRequest(req) {
        for (const interceptor of this.requestInterceptors) {
            req = await interceptor(req);
        }
        return req;
    }

async processResponse(res) {
        for (const interceptor of this.responseInterceptors) {
            res = await interceptor(res);
        }
        return res;
    }
}

// Express middleware
const interceptor = new Interceptor();

// Add auth token to all requests
interceptor.addRequestInterceptor(async (req) => {
    const token = await getAuthToken();
    req.headers = req.headers || {};
    req.headers.Authorization = `Bearer ${token}`;
    return req;
});

// Log all responses
interceptor.addResponseInterceptor(async (res) => {
    console.log("Response:", res.status, res.data);
    return res;
});

app.use(async (req, res, next) => {
    const processedReq = await interceptor.processRequest(req);
    const originalJson = res.json.bind(res);
    res.json = async (data) => {
        const processedRes = await interceptor.processResponse({
            status: res.statusCode,
            data,
        });
        return originalJson(processedRes.data);
    };
    next();
});
```

Node.js: How do you implement a custom request/response interceptor?