Express middleware to configure response

Question

I want to create a middleware that automatically formats my output and returns it in a format that looks like

{
   "successful": "true",
   "message": "Successfully created",
   "data": {
         "name": "Joe",
         "year": 1
    }
}

Currently I'm just returning a json of the data itself (name, year, etc.)

I want to add the "successful", "message", etc.

Here's some snippets of my code below:

routes/student.js

var student_controller = require('../controllers/studentController');
router.get('/list', student_controller.student_list);

controllers/student.js

var Student = require('../models/student');

exports.student_list = function(req, res, next) {
    Student.find()
        .exec(function(err, list_students) {
            if (err) {return next(err);}
            res.json(list_students);
        });
};

app.js

var studentRouter = require('./routes/student');
app.use('/student', studentRouter);

How do I make this middleware, and in which file(s) should I be calling it?

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. -----> Write your logic to add success wrapper around the response
                    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.