打字稿-条件映射类型

Question

I would like to map a plain old Javascript type to an equivalent one using Knockout observables.

interface KnockoutObservableArray<T> { }
interface KnockoutObservable<T> { }

class CustomerDto {
    Name: string;
    PetLicenseNumbers: number[];
}

// manually mapped to Knockout-equivalent 
class CustomerDtoKo {
    Name: KnockoutObservable<string>;
    PetLicenseNumbers: KnockoutObservableArray<number>;
}

I'm trying to use mapped types to do the above automatically but I couldn't find a proper way to conditionally map arrays to KnockoutObservableArray<U> while mapping the scalars to KnockoutObservable<U>

type KnockoutType<T> = {
    //[K in keyof T] : KnockoutObservableArray<T[K][0]>
    [K in keyof T] : KnockoutObservable<T[K]>
}

type CustomerDtoKo2 = KnockoutType<CustomerDto>;
// gives
type CustomerDtoKo2 = {
    Name: KnockoutObservable<string>,
    PetLicenseNumbers: KnockoutObservable<number[]> // should be KnockoutObservableArray<number>
}

Answer 1

As you expect, conditional types will help you:

type KnockoutType<T> = {
    [K in keyof T]: T[K] extends Array<infer U> ? 
      KnockoutObservableArray<U> : KnockoutObservable<T[K]>
}

Good luck!