Testing TypeScript — Vitest and Type Testing

Testing a TypeScript codebase has two complementary halves: runtime testing, which verifies your code behaves correctly when it runs, and type testing, which verifies your types themselves are correct — that a utility produces the expected type, that a function rejects bad arguments, that an inferred type matches what you intended. Vitest is the modern, TypeScript-native test runner that handles the runtime half with first-class TypeScript support, while type-testing tools (`expectTypeOf`, `tsd`, or the `Equal`/`Expect` pattern) handle the type half. This matters because a typed codebase has type-level logic — generics, conditional types, utility types — that can be just as buggy as runtime logic, yet ordinary tests never exercise it; a `DeepReadonly` that silently produces the wrong type passes every runtime test while being broken. Testing both halves means your behaviour and your type contracts are both verified, which is what gives real confidence in a codebase where the types do meaningful work. Understanding how to test each half is essential for shipping TypeScript that is correct in both dimensions.