← All tasksTypeScript · Middle

Mapped types: MyPick, MyReadonly, MyOmit

Write your own versions of the utility types without the built-in Pick, Readonly, Omit and Exclude.

MyPick<T, K> keeps only the keys K of T (and rejects a key that T does not have), MyReadonly<T> makes every field readonly on the first level, and MyOmit<T, K> removes the keys K while keeping the other fields optional where they were.

type MyPick<T, K> = any;

type MyReadonly<T> = any;

type MyOmit<T, K> = any;

Ctrl+Enter runs the tests. Tab indents; to leave the editor with the keyboard, press Esc and then Tab. Your code is saved in this browser.

Tests (5)

Type tests are checked by the TypeScript compiler in strict mode. Expect<Equal<A, B>> passes when the types match exactly, and // @ts-expect-error passes when the next line does not compile. The first run downloads the compiler, so it takes a few seconds.

types

interface Todo { title: string; done: boolean; tags: string[] }
type _ = Expect<Equal<MyPick<Todo, 'title' | 'done'>, { title: string; done: boolean }>>;

types

interface Todo { title: string; done: boolean }
// @ts-expect-error - 'nope' is not a key of Todo
type _ = MyPick<Todo, 'nope'>;

types

interface Todo { title: string; meta: { done: boolean } }
type _ = Expect<Equal<MyReadonly<Todo>, { readonly title: string; readonly meta: { done: boolean } }>>;

types

interface Todo { title: string; done: boolean; tags: string[] }
type _ = Expect<Equal<MyOmit<Todo, 'tags'>, { title: string; done: boolean }>>;

types

interface Todo { title: string; done?: boolean }
type _ = Expect<Equal<MyOmit<Todo, 'title'>, { done?: boolean }>>;