Main.js

const UserRepository = require('./UserRepository.js');
const UserService = require('./UserService.js');
const knex = require('knex');
const express = require('express');

const app = express();
const db = knex({
  client: 'mysql',
  connection: {
    host: 'localhost',
    user: 'sample',
    password: 'secret',
    database: 'sample',
  },
});

const userRepository = new UserRepository(db);
const userService = new UserService(userRepository);

app.get('/users/:user_id', (request, response) => {
  const userId = request.params.user_id;
  
  userService.first(userId)
    .then((user) => {
      if (user) {
        return response.send(user);
      }
    
      response.status(404).send({
        error: 'User not found!',
      });
    });
});

app.listen(3000, () => console.log('Server running at port 3000'));

UserRepository.js

class UserRepository {
  constructor(db) {
    this.db = db;
  }

  first(userId) {
    return this.db('users')
      .where('id', userId)
      .then(rows => rows[0]);
  }
}

module.exports = UserRepository;

UserService.js

class UserService {
  constructor(userRepository) {
    this.users = userRepository;
  }

  first(userId) {
    return users.first(userId);
  }
}

module.exports = UserService;