การทดสอบชนิดด้วย expectTypeOf และ tsd

import { expectTypeOf } from 'vitest';
expectTypeOf(42).toBeNumber();
expectTypeOf('hello').toBeString();

import { identity } from '../src/utils';
expectTypeOf(identity(42)).toEqualTypeOf<number>();
expectTypeOf(identity('hi')).toEqualTypeOf<string>();

npm install --save-dev tsd

import { expectType, expectError } from 'tsd';
import { add } from '../src';
expectType<number>(add(1, 2));
expectError(add('a', 'b'));

import { expectTypeOf } from 'expect-type';
expectTypeOf<User>().toHaveProperty('name').toBeString();

type IsArray<T> = T extends any[] ? true : false;
expectTypeOf<IsArray<string[]>>().toEqualTypeOf<true>();
expectTypeOf<IsArray<string>>().toEqualTypeOf<false>();

type Mutable = { -readonly [K in keyof ReadonlyUser]: ReadonlyUser[K] };
expectTypeOf<Mutable['name']>().toBeString();
expectTypeOf<Mutable>().not.toMatchTypeOf<Readonly<Mutable>>();

// package.json
{ "scripts": { "test:types": "tsd" } }