TypeScript strictness that pays rent

Model the impossible states out of existence

Discriminated unions over boolean soup:

// ❌ 4 boolean fields, 16 combos, 12 impossible
interface Request {
  loading: boolean
  error?: Error
  data?: User[]
  refreshing: boolean
}

// ✅ 3 combos, all legal
type Request =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'error'; error: Error }
  | { status: 'success'; data: User[]; fetchedAt: number }

Switch on the discriminant and never-check exhaustiveness:

function render(r: Request) {
  switch (r.status) {
    case 'idle': return null
    case 'loading': return <Spinner/>
    case 'error': return <Error err={r.error}/>
    case 'success': return <List data={r.data}/>
    default: {
      const _exhaustive: never = r
      return _exhaustive
    }
  }
}

Narrow, don't assert

• assert/as silences the compiler; satisfies/guards teach it:

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

• satisfies keeps inference while checking shape: const config = {...} satisfies Config.

Branded types at the boundaries

IDs, emails, money amounts are not string/number:

type UserId = string & { readonly __brand: 'UserId' }
const asUserId = (s: string) => s as UserId
// loadUser(string) no longer compiles with an order id

Use at module boundaries (API clients, DB access) where mix-ups are catastrophic and cheap to prevent.

Generics: only when the caller gains information

• A generic that appears once and is unrelated to other params is noise.
• Useful generics correlate inputs/outputs: function pick<T, K extends keyof T>(obj: T, keys: K[]): Pick<T, K>.

any triage

• unknown + narrowing for external input.
• as const for literal maps and route tables.
• If a real any remains, quarantine it in one adapter function with a comment stating the invariant the compiler can't see.

Migration playbook (strict on an existing repo)

1. tsc --strict → list errors by rule.
2. Fix strictNullChecks first (biggest payoff), then noImplicitAny.
3. For legacy seas of any, add // @ts-expect-error with reason — it errors when the code becomes correct, unlike plain suppression.

Definition of done (agent checklist)

• No new any; unknown at external boundaries
• Unions use discriminants; exhaustiveness via never
• IDs/emails/money branded at boundaries
• No type assertions across unrelated types
• Public function params/returns explicitly typed