TypeScript type testing util less than in 40 lines

zippy-type-predicates.ts

// Copyright 2022 Masaki Hara
// Licensed in MIT https://opensource.org/licenses/MIT

type Predicate<T> = {
  co?: (() => T) | undefined;
  contra?: ((x: T) => void) | undefined;
};

function expectType<T>() {
  return {
    to(_pred: Predicate<T>): void {}
  };
}

type AssignableToPredicate<T> = {
  co?: undefined;
  contra(x: T): void;
};
function beAssignableTo<T>(): AssignableToPredicate<T> {
  return { contra(_x: T) {} }
}

type AssignableFromPredicate<T> = {
  co(): T;
  contra?: undefined;
};
function beAssignableFrom<T>(): AssignableFromPredicate<T> {
  return { co() { return null as any; } };
}

type TypeEqualPredicate<T> = {
  co(): T;
  contra(x: T): void;
};
function beTypeEqual<T>(): TypeEqualPredicate<T> {
  return { co() { return null as any; }, contra(_x: T) {} }
}

// Type predicate self check

expectType<number>().to(beAssignableTo<number>());
// @ts-expect-error
expectType<number>().to(beAssignableTo<string>());
expectType<number>().to(beAssignableTo<number | string>());
// @ts-expect-error
expectType<number>().to(beAssignableTo<never>());

expectType<number>().to(beAssignableFrom<number>());
// @ts-expect-error
expectType<number>().to(beAssignableFrom<string>());
// @ts-expect-error
expectType<number>().to(beAssignableFrom<number | string>());
expectType<number>().to(beAssignableFrom<never>());

expectType<number>().to(beTypeEqual<number>());
// @ts-expect-error
expectType<number>().to(beTypeEqual<string>());
// @ts-expect-error
expectType<number>().to(beTypeEqual<number | string>());
// @ts-expect-error
expectType<number>().to(beTypeEqual<never>());