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keyof: pluck

pluck(items, key) returns an array of one field's values: pluck(users, "name") gives ["Ann", "Bob"].

Type it so that key can only be a real field of the objects (a typo is a compile error), and the result has that field's type: string[] for name, number[] for id.

function pluck(items: any[], key: string): any[] {
  // ...
  return [];
}

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Tests (6)

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

const users = [{ id: 1, name: 'Ann' }, { id: 2, name: 'Bob' }];
const names = pluck(users, 'name');
type _ = Expect<Equal<typeof names, string[]>>;

types

const users = [{ id: 1, name: 'Ann' }];
const ids = pluck(users, 'id');
type _ = Expect<Equal<typeof ids, number[]>>;

types

// @ts-expect-error - users have no email
pluck([{ id: 1, name: 'Ann' }], 'email');

const users = [{ id: 1, name: 'Ann' }, { id: 2, name: 'Bob' }];
expect(pluck(users, 'name')).toEqual(['Ann', 'Bob']);

expect(pluck([{ id: 1 }, { id: 2 }], 'id')).toEqual([1, 2]);

○
```
expect(pluck([], 'id')).toEqual([]);
```