What are some TypeScript patterns that make your code safer?

TypeScript offers patterns beyond basic typing that catch entire categories of bugs.

Exhaustive checking ensures you handle all cases. In a switch on a union type, add a default case that assigns to never: default: const _exhaustive: never = status; return _exhaustive. If you add a new union member but forget to handle it, TypeScript errors at compile time.

Branded types add semantic meaning to primitive types. type UserId = string & { readonly brand: unique symbol }. Now you can't accidentally pass a ProductId where a UserId is expected, even though both are strings. Create them with constructor functions that validate the input.

Discriminated unions model states cleanly: type State = { status: 'loading' } | { status: 'success', data: Data } | { status: 'error', error: Error }. TypeScript narrows the type based on the status field, ensuring you only access data when status is 'success'.

Use readonly for data that shouldn't change: readonly arrays, Readonly<T> for objects, as const for literal types. This prevents accidental mutations and documents intent.

Template literal types create precise string types: type Route = `/users/${string}`. These catch typos and enable autocomplete for string-based APIs. Combined with mapped types, they're powerful for type-safe configuration.