typescript

TypeScript

TypeScript is only as safe as its strictness lets it be. With strict off (or any/! sprinkled in) it's JavaScript with extra syntax — the checker is on but blindfolded. The craft is letting the type system prove the bug can't happen, not narrating types after the fact.

Check a tsconfig for the strict flags that actually matter: python3 scripts/check_tsconfig.py tsconfig.json

Strict is the floor

// tsconfig.json — the flags that catch real bugs
{
  "compilerOptions": {
    "strict": true,                       // the umbrella — turn it on, always
    "noUncheckedIndexedAccess": true,     // arr[i] is T | undefined (it really is!)
    "noImplicitOverride": true,
    "exactOptionalPropertyTypes": true,
    "noFallthroughCasesInSwitch": true
  }
}

strict: true enables noImplicitAny, strictNullChecks, and more — without it, null/undefined are assignable everywhere and the #1 class of runtime crash goes uncaught. noUncheckedIndexedAccess is the highest-value non-default: it makes arr[i] honestly T | undefined. Full rationale → references/strictness.md.

unknown, never any

// Wrong — any disables the checker for everything downstream; the bug ships
function parse(json: string): any { return JSON.parse(json); }
const u = parse(s); u.naem.toUpperCase();   // typo compiles, crashes at runtime

// Correct — unknown forces you to narrow before use
function parse(json: string): unknown { return JSON.parse(json); }
const u = parse(s);
if (isUser(u)) u.name.toUpperCase();         // narrowed via a type guard

any is a hole in the type system that spreads — every value derived from an any is any. unknown is the safe top type: you must narrow it before you touch it. Reserve any for genuinely untypable third-party seams, and isolate it behind a typed wrapper.

Discriminated unions over optional-flag soup

// Wrong — every field optional; illegal combinations compile
type Result = { ok?: boolean; data?: User; error?: string };

// Correct — a discriminant makes illegal states unrepresentable
type Result =
  | { status: "ok"; data: User }
  | { status: "error"; error: string };

function render(r: Result) {
  if (r.status === "ok") r.data;     // narrowed — r.error doesn't exist here
  else r.error;                       // exhaustive
}