TypeScript Development

Project Setup

Initialize with pnpm

pnpm init pnpm add -D typescript @types/node

TypeScript config

npx tsc --init

tsconfig.json

{ "compilerOptions": { "target": "ES2022", "module": "NodeNext", "moduleResolution": "NodeNext", "strict": true, "noUncheckedIndexedAccess": true, "noImplicitReturns": true, "esModuleInterop": true, "skipLibCheck": true, "outDir": "./dist" }, "include": ["src/**/*"] }

Type Patterns

Discriminated Unions

type Result<T> = | { success: true; data: T } | { success: false; error: Error };

function handleResult(result: Result<User>) { if (result.success) { console.log(result.data); // User } else { console.error(result.error); // Error } }

Branded Types

type UserId = string & { readonly brand: unique symbol }; type OrderId = string & { readonly brand: unique symbol };

function createUserId(id: string): UserId { return id as UserId; }

Utility Types

// Make all properties optional Partial<User>

// Make all properties required Required<User>

// Pick specific properties Pick<User, 'id' | 'email'>

// Omit specific properties Omit<User, 'password'>

// Make properties readonly Readonly<User>

Error Handling

// Result type pattern type Result<T, E = Error> = | { ok: true; value: T } | { ok: false; error: E };

async function fetchUser(id: string): Promise<Result<User>> { try { const user = await db.users.findById(id); if (!user) { return { ok: false, error: new Error('User not found') }; } return { ok: true, value: user }; } catch (error) { return { ok: false, error: error as Error }; } }

Testing with Vitest

import { describe, test, expect, vi } from 'vitest';

describe('UserService', () => { test('creates user with valid email', async () => { const service = new UserService(mockRepo); const user = await service.create('test@example.com'); expect(user.email).toBe('test@example.com'); });

test('throws on invalid email', async () => { const service = new UserService(mockRepo); await expect(service.create('invalid')).rejects.toThrow(); }); });

Tooling

Biome (linting + formatting)

pnpm add -D @biomejs/biome pnpm biome check --apply .

Vitest (testing)

pnpm add -D vitest pnpm vitest