How do you test TypeScript types?

Runtime tests check values; type tests check the compiler's view. A generic helper can still return the right runtime value while its type silently widens to any or loses a relationship. Type tests make those regressions fail in CI.

type Equal<A, B> =
  (<T>() => T extends A ? 1 : 2) extends
  (<T>() => T extends B ? 1 : 2) ? true : false;

type Expect<T extends true> = T;

type Result = Expect<Equal<ReturnType<() => string>, string>>;

Techniques

• Use a compile-time Equal helper for exact type comparisons
• Use Expect to turn a false comparison into a type error
• Use @ts-expect-error on calls that must be rejected
• Use a library such as tsd or expect-type for a richer API
• Run tsc in CI so type tests are part of the build

A type test should be small and focused. Test the public behavior of a generic helper rather than its internal implementation. If a test needs a deeply nested conditional type to express the expectation, the helper may be too complex for its use case.