TypeScript Strict Mode

Strict TypeScript or nothing. Every any is a bug waiting to happen.

Banned patterns

• any — use unknown and narrow, or define the type
• as SomeType — use type guards or narrowing instead
• !.property — handle null/undefined explicitly
• @ts-ignore — fix the type error, don't suppress it
• Implicit any in function parameters

Required patterns

• Explicit return types on public functions
• unknown over any for truly unknown data (then narrow)
• satisfies over as for type assertions
• Discriminated unions over optional properties
• const assertions for literal types
• Exhaustive switches with never checks

Narrowing toolkit

// Type guard
function isUser(x: unknown): x is User {
  return typeof x === 'object' && x !== null && 'id' in x
}
// Exhaustive switch
function handle(action: Action): never {
  switch (action.type) {
    case 'A': return handleA(action)
    case 'B': return handleB(action)
    default: const _exhaustive: never = action; throw new Error()
  }
}

Config

tsconfig.json must have: "strict": true — this enables all strict checks.