Introduction
Functional programming (FP) has gained popularity for its composability, testability, and robustness. In the JavaScript ecosystem, libraries like fp-ts bring powerful FP concepts to TypeScript, allowing you to write cleaner and more reliable code.
This article explores fp-ts concepts like Option, Either, Task, Reader, and ReaderTaskEither. We’ll build a basic CRUD app using fp-ts, pg (PostgreSQL client), and Express.js to see how these abstractions shine in real-world applications.
Key Concepts
Before diving into the app, let’s briefly discuss the main concepts:
- Option: Models the presence or absence of a value (Some or None).
- Either: Represents computations that can succeed (Right) or fail (Left).
- Task: Represents lazy asynchronous computations.
- Reader: Injects dependencies into computations.
- ReaderTaskEither: Combines Reader, Task, and Either for async operations with dependencies and error handling.
Setting Up the Project
Initialize the Project
mkdir fp-ts-crud && cd fp-ts-crud npm init -y npm install express pg fp-ts io-ts npm install --save-dev typescript @types/express ts-node-dev jest @types/jest ts-jest
Setup TypeScript
Create a tsconfig.json:
{ "compilerOptions": { "target": "ES2020", "module": "CommonJS", "outDir": "dist", "strict": true, "esModuleInterop": true }, "include": ["src/**/*"] }
Project Structure
src/ index.ts # Entry point db.ts # Database setup models/ # Data models and validation services/ # Business logic controllers/ # CRUD operations utils/ # fp-ts utilities errors/ # Custom error classes
Implementing the CRUD App
Database Setup (db.ts)
import { Pool } from 'pg'; export const pool = new Pool({ user: 'postgres', host: 'localhost', database: 'fp_ts_crud', password: 'password', port: 5432, });
Define Models and Validation (models/User.ts)
import * as t from 'io-ts'; import { isRight } from 'fp-ts/Either'; export const User = t.type({ id: t.number, name: t.string, email: t.string, }); export const validateUser = (data: unknown): t.TypeOf<typeof User> | null => { const result = User.decode(data); return isRight(result) ? result.right : null; };
Custom Error Handling (errors/AppError.ts)
export class AppError extends Error { constructor(public statusCode: number, public code: string, public message: string) { super(message); this.name = 'AppError'; } } export const createAppError = (statusCode: number, code: string, message: string): AppError => { return new AppError(statusCode, code, message); };
Service Layer (services/UserService.ts)
import { pool } from '../db'; import { ReaderTaskEither, right, left } from 'fp-ts/ReaderTaskEither'; import { pipe } from 'fp-ts/function'; import { createAppError, AppError } from '../errors/AppError'; type Dependencies = { db: typeof pool }; type User = { name: string; email: string }; export const createUser = ( user: User ): ReaderTaskEither<Dependencies, AppError, string> => (deps) => async () => { try { const result = await deps.db.query( 'INSERT INTO users (name, email) VALUES (, ) RETURNING id', [user.name, user.email] ); return right(`User created with ID: ${result.rows[0].id}`); } catch (error) { return left(createAppError(500, 'USER_CREATION_FAILED', 'Failed to create user')); } }; export const getUser = ( id: number ): ReaderTaskEither<Dependencies, AppError, { id: number; name: string; email: string }> => (deps) => async () => { try { const result = await deps.db.query('SELECT * FROM users WHERE id = ', [id]); return result.rows[0] ? right(result.rows[0]) : left(createAppError(404, 'USER_NOT_FOUND', 'User not found')); } catch { return left(createAppError(500, 'USER_FETCH_FAILED', 'Failed to fetch user')); } };
CRUD Operations (controllers/UserController.ts)
import { pipe } from 'fp-ts/function'; import { createUser, getUser } from '../services/UserService'; import { pool } from '../db'; import { AppError } from '../errors/AppError'; const errorHandler = (err: unknown, res: express.Response): void => { if (err instanceof AppError) { res.status(err.statusCode).json({ error: { code: err.code, message: err.message } }); } else { res.status(500).json({ error: { code: 'UNKNOWN_ERROR', message: 'An unexpected error occurred' } }); } }; export const createUserHandler = (req: express.Request, res: express.Response): void => { pipe( createUser(req.body), (task) => task({ db: pool }), (promise) => promise.then((result) => result._tag === 'Left' ? errorHandler(result.left, res) : res.json({ message: result.right }) ) ); }; export const getUserHandler = (req: express.Request, res: express.Response): void => { pipe( getUser(parseInt(req.params.id, 10)), (task) => task({ db: pool }), (promise) => promise.then((result) => result._tag === 'Left' ? errorHandler(result.left, res) : res.json(result.right) ) ); };
Express API (index.ts)
import express from 'express'; import { createUserHandler, getUserHandler } from './controllers/UserController'; const app = express(); app.use(express.json()); // Routes app.post('/users', createUserHandler); app.get('/users/:id', getUserHandler); // Start Server app.listen(3000, () => { console.log('Server running on http://localhost:3000'); });
Running the App with Docker and Docker Compose
Dockerfile
# Stage 1: Build FROM node:22 AS builder WORKDIR /app COPY package*.json . RUN npm install COPY . . RUN npm run build # Stage 2: Run FROM node:22 WORKDIR /app COPY --from=builder /app/dist ./dist COPY package*.json ./ RUN npm install --production CMD ["node", "dist/index.js"]
docker-compose.yml
version: '3.8' services: db: image: postgres:15 environment: POSTGRES_USER: postgres POSTGRES_PASSWORD: password POSTGRES_DB: fp_ts_crud ports: - "5432:5432" volumes: - db_data:/var/lib/postgresql/data volumes: db_data:
Run the App - Development Mode
# Start the database docker-compose up -d # Run the app npx ts-node-dev src/index.ts
Run the App - Production Mode
# Build the docker image docker build -t fp-ts-crud-app . # Start the database docker-compose up -d # Run the container docker run -p 3000:3000 fp-ts-crud-app
Writing Tests
Setup Jest
Update package.json scripts:
mkdir fp-ts-crud && cd fp-ts-crud npm init -y npm install express pg fp-ts io-ts npm install --save-dev typescript @types/express ts-node-dev jest @types/jest ts-jest
Example Test (__tests__/UserService.test.ts)
{ "compilerOptions": { "target": "ES2020", "module": "CommonJS", "outDir": "dist", "strict": true, "esModuleInterop": true }, "include": ["src/**/*"] }
Conclusion
By leveraging fp-ts, Docker, and robust error handling, we built a functional, scalable, and maintainable Node.js CRUD application. Using functional programming patterns makes your code more predictable and reliable, especially when handling asynchronous workflows.
The above is the detailed content of Functional Programming with fp-ts in Node.js. For more information, please follow other related articles on the PHP Chinese website!

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