server.js

const express = require('express');
const next = require('next');

const dev = process.env.NODE_ENV !== 'production';
const app = next({ dev });
const handle = app.getRequestHandler();

const i18nextMiddleware = require('i18next-express-middleware');
const Backend = require('i18next-node-fs-backend');
const i18n = require('./i18n');

const port = process.env.PORT || 3000;
// init i18next with serverside settings
// using i18next-express-middleware
i18n
    .use(Backend)
    .use(i18nextMiddleware.LanguageDetector)
    .init(
        {
            preload: ['en'], // preload all langages
            ns: ['common', 'home'], // need to preload all the namespaces
            backend: {
                loadPath: __dirname + '/locales/{{lng}}/{{ns}}.json',
                addPath: __dirname + '/locales/{{lng}}/{{ns}}.missing.json'
            }
        }, 
        () => {
            // loaded translations we can bootstrap our routes
            app.prepare()
                .then(() => {
                    const server = express();
        
                    // enable middleware for i18next
                    server.use(i18nextMiddleware.handle(i18n));
        
                    // serve locales for client
                    server.use('/locales', express.static(__dirname + '/locales'));
        
                    // missing keys
                    server.post('/locales/add/:lng/:ns', i18nextMiddleware.missingKeyHandler(i18n));
        
                    // use next.js
                    server.get('/blog/p/:id/*', (req, res) => {
                        const actualPage = '/post';
                        const queryParams = { id: req.params.id};
                        app.render(req, res, actualPage, queryParams);
                    });
                    server.get('*', (req, res) => handle(req, res));
        
                    server.listen(port, (err) => {
                        if (err) throw err;
                        //eslint-disable-next-line no-console
                        console.log(`> Ready on http://localhost:${port}`);
                    });
                });
    
        }
    );