ts-typeslisted

# TypeScript Types Good types are constraints, not descriptions. The goal is to make invalid states unrepresentable — so the compiler catches mistakes before they reach runtime. ## Prefer `unknown` over `any` `any` disables type checking. `unknown` preserves it. ```ts // Bad: silences all errors downstream function parse(raw: any) { return raw.data.value; // no error, even if raw is null } // Good: forces the caller to narrow before using function parse(raw: unknown) { if (typeof raw === 'object' && raw !== null && 'data' in raw) { // now safe to use } } ``` Use `any` only at integration boundaries with untyped third-party code, and isolate it — don't let it propagate. ## Avoid type assertions (`as`) `as T` tells the compiler to trust you. It doesn't check. Use it only when you genuinely know something the compiler can't infer, and add a comment explaining why. ```ts // Bad: silences a real error const el = document.getElementById('app') as HTMLDivElement; // Better: narrow properly const el = document.getElementById('app'); if (!(el instanceof HTMLDivElement)) throw new Error('Missing #app'); ``` ## Discriminated unions for modeling state When a value can be in multiple mutually exclusive states, use a discriminated union — a shared literal field that TypeScript uses to narrow. ```ts type Result<T> = | { status: 'ok'; data: T } | { status: 'error'; message: string } | { status: 'loading' }; function render(result: Result<User>) { if (result.st