There is the use-case of testing if only certain TS types cover a special type definition.
// a type that reflects functions and arrow functions
type Fn<A extends any[] = any[], R = any> = (...args: A) => R;
I need to ensure that only and only function cover the Fn type. E.g. type Fn = any would cover functions but also other types that are not intended to be expected. The existing type Equals is unsuitable for testing if types are inside or outside of a domain (read: a set of valid types).
Introducing an Extends type to tsafe would make it possible to assert that Fn covers the types we intend to cover and to ensure that other types are not covered (as expected).
type Extends<T1, T2> = T1 extends T2 ? true : false;
By having the following unit tests, we would ensure that the Fn type does not cover more types than expected.
import { assert } from "tsafe/assert";
import { Extends, Not } from "tsafe";
type Fn<A extends any[] = any[], R = any> = (...args: A) => R;
function fn() {}
class A {}
assert<Extends<() => any, Fn>>();
assert<Extends<() => void, Fn>>();
assert<Extends<(...args: any[]) => any, Fn>>();
assert<Extends<typeof fn, Fn>>();
assert<Not<Extends<undefined, Fn>>>();
assert<Not<Extends<null, Fn>>>();
assert<Not<Extends<boolean, Fn>>>();
assert<Not<Extends<number, Fn>>>();
assert<Not<Extends<string, Fn>>>();
assert<Not<Extends<any[], Fn>>>();
assert<Not<Extends<object, Fn>>>();
assert<Not<Extends<A, Fn>>>();