typescript-best-practices

TypeScript Best Practices

Guidance for writing type-safe, maintainable TypeScript code. Covers type safety fundamentals, async/await patterns, and null handling strategies.

Core Principles

1. Strict mode always: Enable strict: true in tsconfig.json
2. Explicit over implicit: Prefer explicit types for public APIs
3. Narrow types: Use discriminated unions over loose object types
4. Null safety: Handle null/undefined explicitly with strict null checks
5. Immutability: Prefer readonly and const where possible

Type Safety

Prefer Interfaces for Object Shapes

// Good - interfaces are more extensible
interface User {
  id: string;
  name: string;
  email: string;
}

// Use type for unions, intersections, mapped types
type UserRole = 'admin' | 'user' | 'guest';
type UserWithRole = User & { role: UserRole };

Use Discriminated Unions for Variants

// Good - exhaustive pattern matching
type Result<T, E = Error> =
  | { success: true; data: T }
  | { success: false; error: E };

function handleResult<T>(result: Result<T>) {
  if (result.success) {
    return result.data; // TypeScript knows data exists
  }
  throw result.error; // TypeScript knows error exists
}

Avoid any - Use unknown Instead

// Bad
function processData(data: any) {
  return data.value; // No type safety
}

// Good
function processData(data: unknown) {
  if (isValidData(data)) {
    return data.value; // Type narrowed via guard
  }
  throw new Error('Invalid data');
}

function isValidData(data: unknown): data is { value: string } {
  return typeof data === 'object' && data !== null && 'value' in data;
}

Generic Constraints

// Good - constrained generics
function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

// Good - default type parameters
interface Repository<T extends { id: string } = { id: string }> {
  findById(id: string): Promise<T | null>;
  save(entity: T): Promise<T>;
}

Async Patterns

Always Await in Try-Catch

// Good
async function fetchUser(id: string): Promise<User> {
  try {
    const response = await fetch(`/api/users/${id}`);
    if (!response.ok) {
      throw new Error(`HTTP ${response.status}`);
    }
    return await response.json();
  } catch (error) {
    console.error('Failed to fetch user:', error);
    throw error;
  }
}

Use Promise.all for Parallel Operations

// Good - parallel execution
async function fetchUserData(userId: string) {
  const [user, posts, comments] = await Promise.all([
    fetchUser(userId),
    fetchUserPosts(userId),
    fetchUserComments(userId),
  ]);
  return { user, posts, comments };
}

Type Async Return Values Explicitly

// Good - explicit return type
async function createUser(data: CreateUserDTO): Promise<User> {
  const user = await db.users.create(data);
  return user;
}

// Good - handle errors with Result type
async function safeCreateUser(data: CreateUserDTO): Promise<Result<User>> {
  try {
    const user = await db.users.create(data);
    return { success: true, data: user };
  } catch (error) {
    return { success: false, error: error as Error };
  }
}

Null Handling

Use Optional Chaining and Nullish Coalescing

// Good
const userName = user?.profile?.name ?? 'Anonymous';
const config = options?.timeout ?? DEFAULT_TIMEOUT;

// Avoid
const userName = user && user.profile && user.profile.name || 'Anonymous';

Non-Null Assertion Only When Certain

// OK when you've verified externally
const element = document.getElementById('root')!;

// Better - explicit check
const element = document.getElementById('root');
if (!element) {
  throw new Error('Root element not found');
}

Type Guards for Runtime Checks

// Good - type guard function
function isDefined<T>(value: T | null | undefined): value is T {
  return value !== null && value !== undefined;
}

// Usage
const values = [1, null, 2, undefined, 3];
const defined = values.filter(isDefined); // number[]

Prefer undefined Over null for Optional

// Good - consistent with optional properties
interface Options {
  timeout?: number; // undefined when not set
  retries?: number;
}

// Avoid mixing null and undefined
interface Options {
  timeout: number | null; // Inconsistent
  retries?: number;
}

Utility Types

Common Utility Type Patterns

// Make all properties optional
type PartialUser = Partial<User>;

// Make all properties required
type RequiredUser = Required<User>;

// Pick specific properties
type UserCredentials = Pick<User, 'email' | 'password'>;

// Omit specific properties
type PublicUser = Omit<User, 'password'>;

// Make properties readonly
type ImmutableUser = Readonly<User>;

// Record for object maps
type UserCache = Record<string, User>;

Extract and Exclude for Union Types

type AllRoles = 'admin' | 'user' | 'guest' | 'superadmin';
type AdminRoles = Extract<AllRoles, 'admin' | 'superadmin'>; // 'admin' | 'superadmin'
type NonAdminRoles = Exclude<AllRoles, 'admin' | 'superadmin'>; // 'user' | 'guest'

Quick Reference

tsconfig.json Essentials

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "noUncheckedIndexedAccess": true
  }
}

Checklist

• strict: true enabled in tsconfig.json
• No any types without documented justification
• All async functions have error handling
• Public APIs have explicit return types
• Discriminated unions for variant types
• Type guards for runtime type checks
• unknown instead of any for unknown data
• Optional chaining for nullable access
• Nullish coalescing for default values