Custom error handling in a REST API using TypeScript, Express.js, Joi validation, and object-oriented programming

Hi, Devs!

Building a robust and maintainable REST API requires careful consideration of error handling, validation, and structure. In this article, we'll explore how to set up a custom error handling mechanism in a REST API using TypeScript, Express.js, Joi for validation, and principles of object-oriented programming (OOP). We'll structure our project as follows:

Scructure:

---src
-----middlewares
--------ErrorHanlder.ts
-----models
--------User.ts
-----repository
--------UserRepository.ts
-----routes
--------UserRouter.ts
-----controllers
--------UserController.ts
-----services
--------UserService.ts
-----validations
--------UserValidation.ts
---app.ts
---server.ts

Setting Up the Project

mkdir rest-api
cd rest-api
npm init -y
npm install express typescript @types/node @types/express ts-node-dev
npx tsc --init

Middleware for Error Handling

// src/middlewares/ErrorHandler.ts
import { Request, Response, NextFunction } from 'express';

class ErrorHandler extends Error {
  statusCode: number;
  message: string;

  constructor(statusCode: number, message: string) {
    super();
    this.statusCode = statusCode;
    this.message = message;
  }
}

const handleError = (err: ErrorHandler, req: Request, res: Response, next: NextFunction) => {
  const { statusCode, message } = err;
  res.status(statusCode).json({
    status: 'error',
    statusCode,
    message,
  });
};

export { ErrorHandler, handleError };

User Model

// src/models/User.ts
export interface User {
  id: number;
  name: string;
  email: string;
}

User Repository

// src/repository/UserRepository.ts
import { User } from '../models/User';

class UserRepository {
  private users: User[] = [];

  findAll(): User[] {
    return this.users;
  }

  findById(id: number): User | undefined {
    return this.users.find(user => user.id === id);
  }

  create(user: User): User {
    this.users.push(user);
    return user;
  }
}

export default new UserRepository();

User Service

// src/services/UserService.ts
import UserRepository from '../repository/UserRepository';
import { User } from '../models/User';
import { ErrorHandler } from '../middlewares/ErrorHandler';

class UserService {
  getAllUsers(): User[] {
    return UserRepository.findAll();
  }

  getUserById(id: number): User | undefined {
    const user = UserRepository.findById(id);
    if (!user) {
      throw new ErrorHandler(404, 'User not found');
    }
    return user;
  }

  createUser(user: User): User {
    const { error } = userSchema.validate(user);
    if (error) {
      throw new ErrorHandler(400, error.details[0].message);
    }
    return UserRepository.create(user);
  }
}

export default new UserService();

User Validation

// src/validations/UserValidation.ts
import Joi from 'joi';

const userSchema = Joi.object({
  id: Joi.number().required(),
  name: Joi.string().required(),
  email: Joi.string().email().required(),
});

export { userSchema };

User Controller

// src/controllers/UserController.ts
import { Request, Response, NextFunction } from 'express';
import UserService from '../services/UserService';
import { ErrorHandler } from '../middlewares/ErrorHandler';

class UserController {
  getAllUsers(req: Request, res: Response, next: NextFunction) {
    try {
      const users = UserService.getAllUsers();
      res.json(users);
    } catch (error) {
      next(error);
    }
  }

  getUserById(req: Request, res: Response, next: NextFunction) {
    try {
      const user = UserService.getUserById(parseInt(req.params.id));
      res.json(user);
    } catch (error) {
      next(error);
    }
  }

  createUser(req: Request, res: Response, next: NextFunction) {
    try {
      const user = UserService.createUser(req.body);
      res.status(201).json(user);
    } catch (error) {
      next(error);
    }
  }
}

export default new UserController();

User Routes

// src/routes/UserRouter.ts
import { Router } from 'express';
import UserController from '../controllers/UserController';

const router = Router();

router.get('/users', UserController.getAllUsers);
router.get('/users/:id', UserController.getUserById);
router.post('/users', UserController.createUser);

export default router;

Application Entry Point

// src/app.ts
import express from 'express';
import UserRouter from './routes/UserRouter';
import { handleError } from './middlewares/ErrorHandler';

const app = express();

app.use(express.json());
app.use('/api', UserRouter);
app.use(handleError);

export default app;

Server Setup

// src/server.ts
import app from './app';

const PORT = process.env.PORT || 3000;

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

By structuring our project using TypeScript, Express.js, Joi, and OOP principles, we achieve a clean and maintainable codebase. The custom error handling middleware provides a consistent way to manage errors across the application, while the validation logic ensures data integrity. This setup can serve as a solid foundation for more complex applications.

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