Extending a Type to Create Child Type

Question

I am working with TypeScript and am wondering how to achieve a child type of another. I don't know exactly how to describe what I am after, so let me just show an example.

type Person = {
 name: string;
 type: PersonType
}

type Child = Person & {
 type: PersonType.child,
}

enum PersonType {
 adult = "adult",
 child = "child",
}

const people: Person[] = [...];
const children: Child[] = people.filter(p => p.type === PersonType.child);

Is this something I would use a generic for? I would think this is straight forward and I'm just missing a piece. Thanks!

Answer 1

You added redundant = sign to enum.

type Person = {
    name: string;
    type: PersonType
}

type Child = Person & {
    type: PersonType.child,
}

enum PersonType {
    adult = "adult",
    child = "child",
}

type TestChild = Person extends Child ? true : false // false
type TestPerson = Child extends Person ? true : false // true


const people: Person[] = [{ name: 'John', type: PersonType.adult }, { name: 'Tim', type: PersonType.child }];
const children: Child[] = people.filter(p => p.type === PersonType.child);

The problem is that Child extends Person, but not vice versa. You can't just use explicitly type Child[] for children constant, because Person does not extends Child

Here is interesting article about covariance.

But, you can achieve desired behaviour, just use filter as a typeguard:

const children: Child[] = people.filter((p):p is Child => p.type === PersonType.child); // ok

More interesting examples, You can find here