How to get response body in middleware?

Question

I want to log every request with response information.

I tried to use middleware, and I have a problem.

res.body is undefined.

app.use((req, res, next) => {

    var time = Date.now();

    res.on('finish', function() {
        var clientIp = req.headers['x-forwarded-for'] || req.connection.remoteAddress;
        var method = req.method;
        var path = req.baseUrl;
        var duration = Date.now() - time;

        console.log({
            clientIp,
            elapsedTime: `${duration}ms`,
            hostname: req.headers.host,
            level: 'INFO',
            method,
            path,
            phase: process.env.NODE_ENV,
            reqBody: req.body,
            reqHeaders: req.headers,
            resBody: res.body,
            resHeaders: res.getHeaders(),
            status: res.statusCode
        });
    });

    next();
});

Actually there is response data on client.

How can I get response body in middleware?

Answer 1

The response can be intercepted by overriding response.json function. By doing so, and adding our custom function, every time, response.json() is called, our intercepting function is triggered.

middleware/response.filter.js:

// Response Interceptor Middleware
export default (request, response, next) => {
    try {
        const oldJSON = response.json;
        response.json = (data) => {
            // For Async call, handle the promise and then set the data to `oldJson`
            if (data && data.then != undefined) {
                // Resetting json to original to avoid cyclic call.
                return data.then((responseData) => {
                    // Custom logic/code.
                    response.json = oldJSON;
                    return oldJSON.call(response, responseData);
                }).catch((error) => {
                    next(error);
                });
            } else {
                // For non-async interceptor functions
                // Resetting json to original to avoid cyclic call.
                // Custom logic/code.
                response.json = oldJSON;
                return oldJSON.call(response, finalResponse);
            }
        }
    } catch (error) {
        next(error);
    }
}

In the Server.js file, register the middleware:

// Server.js file
import externalResponseFilter from "./middleware/response.filter.js:";

// Create Express server
const app = express();

// Response interceptor - Initialization.
app.use(externalResponseFilter);

And in the controller where you are returning the response, return with response.json() function instead of response.send().

Let me know if any additional explanation is required.