Advanced TypeScript Patterns: Branded Types, Discriminated Unions, and Exhaustive Checks

TypeScript's type system is powerful but most devs only scratch the surface. These patterns eliminate entire categories of runtime bugs by making invalid states unrepresentable.

1. Discriminated Unions for State Machines

Instead of boolean flags that can conflict:

// Bad -- 'loading' and 'data' can both be true
interface State {
  loading: boolean
  data: User | null
  error: Error | null
}

// Good -- only one state at a time
type AsyncState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error }

// TypeScript narrows automatically
function render(state: AsyncState<User>) {
  if (state.status === 'success') {
    return state.data.name // TypeScript knows data exists
  }
  if (state.status === 'error') {
    return state.error.message // TypeScript knows error exists
  }
}

2. Branded Types

Prevent mixing semantically different strings/numbers:

type UserId = string & { readonly _brand: 'UserId' }
type PostId = string & { readonly _brand: 'PostId' }

function makeUserId(id: string): UserId {
  return id as UserId
}

function getUser(id: UserId) { /* ... */ }
function getPost(id: PostId) { /* ... */ }

const userId = makeUserId('user_123')
const postId = 'post_456' as PostId

getUser(userId) // OK
getUser(postId) // Type error -- caught at compile time

Useful for: IDs, currencies, validated strings (emails, URLs).

3. Template Literal Types

Type-safe string patterns:

type EventName = `on${Capitalize<string>}`
type CSSProperty = `--${string}`
type ApiRoute = `/api/${string}`

type HttpMethod = 'GET' | 'POST' | 'PUT' | 'DELETE' | 'PATCH'
type ApiEndpoint = `${HttpMethod} /api/${string}`

// Usage
function registerRoute(endpoint: ApiEndpoint) { /* ... */ }

registerRoute('GET /api/users')    // OK
registerRoute('INVALID /api/foo')  // Type error
registerRoute('GET /other/path')   // Type error

4. Conditional Types for Utility Types

// Extract the resolved type from a Promise
type Awaited<T> = T extends Promise<infer U> ? U : T

// Make specific keys required
type RequireKeys<T, K extends keyof T> = T & Required<Pick<T, K>>

// Deep partial
type DeepPartial<T> = {
  [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K]
}

// Strict omit that errors on invalid keys
type StrictOmit<T, K extends keyof T> = Omit<T, K>

// Non-nullable deep
type DeepNonNullable<T> = {
  [K in keyof T]: NonNullable<T[K]>
}

5. Satisfies Operator

Get type checking without losing inference:

type Config = {
  [key: string]: string | number | boolean
}

// With 'as Config' -- loses specific types
const config = {
  name: 'MyApp',
  port: 3000,
} as Config
config.port // type: string | number | boolean

// With 'satisfies' -- validates shape, preserves types
const config2 = {
  name: 'MyApp',
  port: 3000,
} satisfies Config
config2.port // type: number
config2.name // type: string

6. Const Assertions for Literal Types

// Without const -- widened to string[]
const roles = ['admin', 'user', 'guest']
type Role = typeof roles[number] // string

// With const -- literal union
const ROLES = ['admin', 'user', 'guest'] as const
type Role = typeof ROLES[number] // 'admin' | 'user' | 'guest'

function assignRole(role: Role) { /* ... */ }
assignRole('admin')   // OK
assignRole('manager') // Type error

7. Function Overloads

Different return types based on input:

function parse(value: string): number
function parse(value: string[]): number[]
function parse(value: string | string[]): number | number[] {
  if (Array.isArray(value)) {
    return value.map(Number)
  }
  return Number(value)
}

const single = parse('42')           // type: number
const multiple = parse(['1', '2'])   // type: number[]

8. Exhaustive Switch Checks

Catch unhandled cases at compile time:

type Status = 'active' | 'inactive' | 'pending'

function assertNever(x: never): never {
  throw new Error('Unexpected value: ' + x)
}

function getLabel(status: Status): string {
  switch (status) {
    case 'active': return 'Active'
    case 'inactive': return 'Inactive'
    case 'pending': return 'Pending'
    default: return assertNever(status) // Error if you add a new Status
  }
}
// If you add 'suspended' to Status, TypeScript errors on the default case

Putting It Together

These patterns work together. A well-typed API client:

type UserId = string & { _brand: 'UserId' }
type ApiRoute = `/api/${string}`

type ApiResponse<T> =
  | { ok: true; data: T }
  | { ok: false; error: string; code: number }

async function apiGet<T>(route: ApiRoute): Promise<ApiResponse<T>> {
  const r = await fetch(route)
  if (!r.ok) return { ok: false, error: await r.text(), code: r.status }
  return { ok: true, data: await r.json() }
}

// Usage -- fully type-safe
const result = await apiGet<User>('/api/users')
if (result.ok) {
  console.log(result.data.name) // TypeScript knows data: User
} else {
  console.error(result.error)   // TypeScript knows error: string
}

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