How to overwrite property for intersection type in Typescript?

Question

Let's say I have these types:

type BaseAnimal = {
  species: string
  owner: boolean
}

type Cat = BaseAnimal & {
  species: 'cat'
  hasTail: boolean
}

type Dog = BaseAnimal & {
  species: 'dog'
  likesWalks: boolean
}

type Animal = Cat | Dog

And I want to create a type called AnimalParams, which is identical to Animal except the owner property, which is a string.

I can't do either of the below.

// This seems to keep the owner property from Animal instead of overwriting
// So it raises an error if I try to specify owner as a string
type AnimalParams = Animal & {
  owner: string
}

// This strips away properties unique to Cat or Dog
// So it raises an error if I try to specify hasTail or likesWalks
type AnimalParams = Omit<Animal, 'owner'> & {
  owner: string
}

Now, the only workaround I can think of is to do as below, but this seems unnecessarily repetitive. Is there a cleaner, more concise way?

type CatParams = Omit<Cat, 'owner'> & {
  owner: string
}

type DogParams = Omit<Dog, 'owner'> & {
  owner: string
}

type AnimalParams = CatParams | DogParams

I read a few SO threads on utility types (such as Overriding interface property type defined in Typescript d.ts file, which was for interfaces), but couldn't find what I needed. Thanks for any answers in advance!

Answer 1

Instead of manually omitting owner prop from each type, you can use distributive conditional type:

type OmitOwner<T = Animal> = T extends BaseAnimal ? Omit<T, 'owner'> : never;

type AnimalParams = OmitOwner & {
  owner: string
};

Which is equivalent to:

(Omit<Cat, 'owner'> & { owner: string; }) 
  | (Omit<Dog, 'owner'> & { owner: string; })

That's due to automatic distribution over union types

Instantiation of T extends U ? X : Y with the type argument A | B | C for T is resolved as (A extends U ? X : Y) | (B extends U ? X : Y) | (C extends U ? X : Y)

Playground

---

Why original attempt doesn't work?

keyof union produces intersection of keys of types in union, so

type AnimalKeys = keyof Animal // is "species" | "owner"

And implementation of Omit is:

type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;