[Typescript] Testing type

1. TSD: https://github.com/SamVerschueren/tsd

import {expectType} from 'tsd';
import concat from '.';

expectType<string>(concat('foo', 'bar'));
expectType<string>(concat(1, 2));

2. Vitest: https://vitest.dev/guide/testing-types.html

import { assertType, expectTypeOf } from 'vitest'
import { mount } from './mount.js'

test('my types work properly', () => {
  expectTypeOf(mount).toBeFunction()
  expectTypeOf(mount).parameter(0).toMatchTypeOf<{ name: string }>()

  // @ts-expect-error name is a string
  assertType(mount({ name: 42 }))
})

3. Small utils type helpers:

type Expect<T extends true> = T;
type Equal<X, Y> = Expect<X extends Y ? (Y extends X ? true : false) : false>;

// Example

const identityFunc = <T>(arg: T) => arg;

it("Should return whatever you pass in", () => {
  const test1 = identityFunc("hello");

  type test = Expect<Equal<typeof test1, "hello">>;
});

Refer to: https://www.totaltypescript.com/how-to-test-your-types