Let's say that I have 3 types and a union type of them:
type Cat = {
owner: string;
color: string;
lives: number;
}
type Dog = {
owner: string;
color: string;
}
type Wolf = {
color: string;
}
type Animals = Cat | Dog | Wolf;
I want a filter AnimalsFilter<U, K> that would work like this:
type FilterAnimals<U, K extends string> = ...;
type WithOwner = FilterAnimals<Animals, 'owner'>;
// WithOwner = Cat | Dog
So that after filtering this union type it would only have Cat and Dog. How can I achieve that?
I tried mapping, like this:
type FilterAnimals<U, K extends string> = {
[Key in keyof U]: Key extends K? U[K] : never;
}[keyof U];
But it does not work as intended. Am I even close to answer with this?
You can write HasKey<T, K> as a distributive conditional type to split the union type T into its members, then perform another conditional check via keyof to just keep those members with a key from K. Like this:
type HasKey<T, K extends PropertyKey> =
T extends unknown ? K extends keyof T ? T : never : never;
type AnimalsWithOwner = HasKey<Animals, "owner">;
// type AnimalsWithOwner = Cat | Dog