I wrote a blog post about implementing utility types by ourselves. To check if the two types are the same, the expectTypeOf API in Vitest is very convenient.
// In utility-type.test-d.tsimport { describe, expectTypeOf, it } from 'vitest';describe('Pick2', () => { it('should pick type of properties specified in second argument from type of first argument', () => { // The utility type you want to practice type Pick2<T, U extends keyof T> = { [K in U]: T[K]; }; // The target type type Animal = { name: string; age: number; }; // Execution type Actual = Pick2<Animal, 'name'>; type Expected = Pick<Animal, 'name'>; // Assert the custom type utility with built-in type expectTypeOf<Actual>().toEqualTypeOf<Expected>(); });});It will fail if you make a mistake, but pass if your implementation is correct.
$ npx vitest --typecheckFAIL server src/utility-type.test-d.ts > Pick2 > should pick type of properties specified in second argument from type of first argumentTypeCheckError: Type 'Pick<Animal, "name">' does not satisfy the constraint '{ name "Expected string, Actual never"; }'.Types of property 'name' are incompatible.Type 'string' is not assignable to type '"Expected string, Actual never"'.❯ src/utility-type.test-d.ts:19:40 17| 18| // Assert custom type utility with built-in type 19| expectTypeOf<Actual>().toEqualTypeOf<Expected>(); 20| });
