IsMutuallyAssignable

type alias · experimental

type IsMutuallyAssignable<A, E> = [E] extends [A] ? [A] extends [E] ? true : false : false

Checks if the actual type A is assignable to the expected type E, and vice versa.

This is often less strict than IsExact because the two type parameters are allowed to have a different structure as long as they are assignable to each other. This is often more strict than Has because none of the two type parameters may be a union that contains the other, as this would fail the check for mutual assignability.

import { assertType } from "@ende/testing/types";
import type { IsMutuallyAssignable } from "@ende/testing/unstable-types";

// false because E is not assignable to A
assertType<IsMutuallyAssignable<string & RegExpMatchArray, string>>(false);
// false because A is not assignable to E
assertType<IsMutuallyAssignable<string | RegExpMatchArray, string>>(false);
// true because both types are assignable to each other
assertType<IsMutuallyAssignable<string | (string & RegExpMatchArray), string>>(true);