TypeScript Best Practices for Large-Scale Applications

After working on several large-scale TypeScript applications, I've collected patterns and practices that help maintain code quality and developer productivity.

1. Enable Strict Mode

Always use strict mode in tsconfig.json:

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitReturns": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noFallthroughCasesInSwitch": true
  }
}

2. Use Branded Types for Type Safety

Prevent mixing up similar primitive types:

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

function getUserById(id: UserId) { /* ... */ }

// This won't compile - ProductId can't be assigned to UserId
const productId = "prod_123" as ProductId
getUserById(productId) // Error!

3. Discriminated Unions for State Management

Model states explicitly:

type LoadingState = 
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: Data }
  | { status: 'error'; error: Error }

function handleState(state: LoadingState) {
  switch (state.status) {
    case 'success':
      return state.data // TypeScript knows data exists
    case 'error':
      return state.error // TypeScript knows error exists
    // ...
  }
}

4. Generic Constraints

Make generics more specific:

interface HasId {
  id: string
}

function findById<T extends HasId>(items: T[], id: string): T | undefined {
  return items.find(item => item.id === id)
}

5. Utility Types

Leverage built-in utility types:

type User = {
  id: string
  name: string
  email: string
  age: number
}

// Only id and name required
type CreateUser = Pick<User, 'id' | 'name'>

// All fields optional
type UpdateUser = Partial<User>

// All fields readonly
type ImmutableUser = Readonly<User>

6. Const Assertions

Preserve literal types:

const config = {
  apiUrl: 'https://api.example.com',
  timeout: 5000,
} as const

// Type is: { readonly apiUrl: "https://api.example.com", readonly timeout: 5000 }

7. Type Guards

Runtime type checking:

function isUser(obj: unknown): obj is User {
  return (
    typeof obj === 'object' &&
    obj !== null &&
    'id' in obj &&
    'name' in obj
  )
}

if (isUser(data)) {
  // TypeScript knows data is User
  console.log(data.name)
}

8. Avoid any

Use unknown instead:

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

// Good
function processData(data: unknown) {
  if (typeof data === 'object' && data !== null && 'value' in data) {
    return data.value
  }
  throw new Error('Invalid data')
}

Conclusion

TypeScript's type system is incredibly powerful when used correctly. These patterns have helped our team catch bugs early and maintain large codebases with confidence.

What are your favorite TypeScript patterns? Let me know on Twitter!