typescript-best-practices

name	typescript-best-practices
description	Guides TypeScript best practices for type safety, code organization, and maintainability. Use this skill when configuring TypeScript projects, deciding on typing strategies, writing async code, or reviewing TypeScript code quality.

TypeScript Best Practices

Comprehensive guide to writing clean, type-safe, and maintainable TypeScript code.

When to Use

• Configuring a new TypeScript project
• Deciding between interface vs type alias
• Writing async/await code
• Reviewing TypeScript code quality
• Avoiding common TypeScript pitfalls

1. Project Configuration

Always enable strict mode for maximum type safety:

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictBindCallApply": true,
    "strictPropertyInitialization": true,
    "noImplicitThis": true,
    "alwaysStrict": true
  }
}

Why strict mode matters:

• Catches bugs at compile time instead of runtime
• Forces explicit handling of null/undefined
• Prevents implicit any types from sneaking in

2. Type System Best Practices

Use Type Inference

Let TypeScript infer types when obvious:

// Good - inference works fine
const name = 'Alice';
const count = 42;
const items = ['a', 'b', 'c'];

// Bad - redundant annotations
const name: string = 'Alice';
const count: number = 42;

Be Explicit for Public APIs

// Good - explicit for function signatures
function calculateTotal(items: CartItem[], taxRate: number): number {
  return items.reduce((sum, item) => sum + item.price, 0) * (1 + taxRate);
}

// Good - explicit for class properties
class UserService {
  private readonly cache: Map<string, User>;

  constructor(private api: ApiClient) {
    this.cache = new Map();
  }
}

Interface vs Type Alias

Use interface for:

• Object shapes that can be extended
• Public API contracts
• Declaration merging needs

interface User {
  id: string;
  name: string;
}

interface Admin extends User {
  permissions: string[];
}

Use type for:

• Unions and intersections
• Tuples
• Mapped types
• Primitive aliases

type Status = 'pending' | 'approved' | 'rejected';
type Point = [number, number];
type ReadonlyUser = Readonly<User>;

Avoid any - Use unknown with Type Guards

// Bad - defeats type checking
function process(data: any) {
  return data.toUpperCase(); // No error, but might crash
}

// Good - use unknown with type guards
function process(data: unknown): string {
  if (typeof data === 'string') {
    return data.toUpperCase();
  }
  throw new Error('Expected string');
}

// Good - use generics for flexibility
function identity<T>(value: T): T {
  return value;
}

3. Code Organization

File Naming Convention

Use lowercase with dots for clarity:

src/
├── user/
│   ├── user.service.ts
│   ├── user.model.ts
│   ├── user.controller.ts
│   └── index.ts          # Barrel file
├── auth/
│   ├── auth.service.ts
│   └── index.ts
└── types/
    └── index.ts

Barrel Files for Clean Exports

// user/index.ts
export { UserService } from './user.service';
export { User, CreateUserDto } from './user.model';
export { UserController } from './user.controller';

// Consumer imports cleanly
import { UserService, User } from './user';

4. Functions Best Practices

Explicit Parameter Types

// Good - clear contract
function greet(name: string, greeting = 'Hello'): string {
  return `${greeting}, ${name}!`;
}

// Good - use rest parameters for variable arguments
function sum(...numbers: number[]): number {
  return numbers.reduce((a, b) => a + b, 0);
}

Single Responsibility

// Bad - function does too much
function processUser(user: User) {
  // validates, transforms, saves, and sends email
}

// Good - split into focused functions
function validateUser(user: User): ValidationResult { ... }
function transformUser(user: User): TransformedUser { ... }
function saveUser(user: TransformedUser): Promise<void> { ... }
function sendWelcomeEmail(user: User): Promise<void> { ... }

Guard Clauses for Early Returns

// Good - guard clauses
function processOrder(order: Order | null): ProcessedOrder {
  if (!order) throw new Error('Order required');
  if (order.items.length === 0) throw new Error('Order must have items');
  if (order.status !== 'pending') throw new Error('Order already processed');

  // Main logic here - no nesting
  return { ...order, status: 'processed' };
}

5. Async/Await Patterns

Always Handle Errors

// Good - explicit error handling
async function fetchUser(id: string): Promise<User> {
  try {
    const response = await api.get(`/users/${id}`);
    return response.data;
  } catch (error) {
    if (error instanceof NotFoundError) {
      throw new UserNotFoundError(id);
    }
    throw error;
  }
}

Use Promise.all for Parallel Operations

// Bad - sequential when parallel is possible
const user = await fetchUser(id);
const orders = await fetchOrders(id);
const preferences = await fetchPreferences(id);

// Good - parallel execution
const [user, orders, preferences] = await Promise.all([
  fetchUser(id),
  fetchOrders(id),
  fetchPreferences(id),
]);

Flatten Async Chains

// Bad - callback hell with async
async function bad() {
  return fetchUser().then(user => {
    return fetchOrders(user.id).then(orders => {
      return processOrders(orders).then(result => {
        return result;
      });
    });
  });
}

// Good - flat async/await
async function good() {
  const user = await fetchUser();
  const orders = await fetchOrders(user.id);
  return processOrders(orders);
}

6. Testing and Quality

Dependency Injection for Testability

interface PaymentGateway {
  charge(amount: number): Promise<boolean>;
}

class PaymentProcessor {
  constructor(private gateway: PaymentGateway) {}

  async processPayment(amount: number): Promise<boolean> {
    if (amount <= 0) throw new Error('Amount must be positive');
    return this.gateway.charge(amount);
  }
}

// Easy to test with mock
const mockGateway: PaymentGateway = {
  charge: jest.fn().mockResolvedValue(true),
};
const processor = new PaymentProcessor(mockGateway);

Type Guards for Runtime Checking

interface Cat {
  meow(): void;
}

interface Dog {
  bark(): void;
}

function isCat(pet: Cat | Dog): pet is Cat {
  return 'meow' in pet;
}

function makeSound(pet: Cat | Dog) {
  if (isCat(pet)) {
    pet.meow(); // TypeScript knows it's Cat
  } else {
    pet.bark(); // TypeScript knows it's Dog
  }
}

7. Performance Considerations

Type-Only Imports

// Good - type stripped at compile time, better tree-shaking
import type { User, Order } from './types';
import { fetchUser } from './api';

// Also good for re-exports
export type { User, Order };

Const Assertions for Literal Types

// Creates readonly tuple with literal types
const colors = ['red', 'green', 'blue'] as const;
type Color = typeof colors[number]; // "red" | "green" | "blue"

// Works for objects too
const config = {
  apiUrl: 'https://api.example.com',
  timeout: 5000,
} as const;

Avoid Excessive Type Complexity

// Bad - deeply nested mapped types slow compilation
type DeepReadonly<T> = {
  readonly [K in keyof T]: T[K] extends object
    ? DeepReadonly<T[K]>
    : T[K];
};

// Consider simpler alternatives or use sparingly

8. Handle Null/Undefined Properly

Optional Chaining and Nullish Coalescing

// Good - safe property access
function getLength(str: string | null): number {
  return str?.length ?? 0;
}

// Good - safe method calls
const result = user?.getProfile?.()?.name ?? 'Anonymous';

// Good - default values only for null/undefined
const port = config.port ?? 3000; // 0 is valid, won't use default

Discriminated Unions for State

type AsyncState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error };

function renderState<T>(state: AsyncState<T>) {
  switch (state.status) {
    case 'idle':
      return 'Ready';
    case 'loading':
      return 'Loading...';
    case 'success':
      return state.data; // TypeScript knows data exists
    case 'error':
      return state.error.message; // TypeScript knows error exists
  }
}

Common Mistakes to Avoid

Mistake	Problem	Solution
Overusing any	Defeats type checking	Use unknown, generics, or proper types
Not using strict mode	Misses many errors	Enable "strict": true
Redundant annotations	Clutters code	Trust type inference
Ignoring union types	Runtime errors	Use type guards
Not handling null	Crashes	Use ?. and ?? operators
Nested conditionals	Hard to read	Use guard clauses

Quick Reference

// Type inference - let TS do the work
const name = 'Alice';

// Explicit for APIs
function greet(name: string): string { ... }

// Unknown over any
function safe(data: unknown) { ... }

// Type-only imports
import type { User } from './types';

// Const assertions
const tuple = [1, 2] as const;

// Null safety
const len = str?.length ?? 0;

// Guard clauses
if (!valid) throw new Error();
// main logic...

References

• W3Schools TypeScript Best Practices
• TypeScript Handbook
• TypeScript Performance Wiki