[Typescript] 24. Medium - Type Lookup

Sometimes, you may want to lookup for a type in a union to by their attributes.

In this challenge, we would like to get the corresponding type by searching for the common type field in the union Cat | Dog. In other words, we will expect to get Dog for LookUp<Dog | Cat, 'dog'> and Cat for LookUp<Dog | Cat, 'cat'> in the following example.

interface Cat {
  type: 'cat'
  breeds: 'Abyssinian' | 'Shorthair' | 'Curl' | 'Bengal'
}

interface Dog {
  type: 'dog'
  breeds: 'Hound' | 'Brittany' | 'Bulldog' | 'Boxer'
  color: 'brown' | 'white' | 'black'
}

type MyDogType = LookUp<Cat | Dog, 'dog'> // expected to be `Dog`

 

interface Cat {
  type: 'cat'
  breeds: 'Abyssinian' | 'Shorthair' | 'Curl' | 'Bengal'
}
type x = Cat extends {type: 'cat'} ? true: false // true

 

/* _____________ Your Code Here _____________ */

type LookUp<T extends {type: string}, Kind extends string> = T extends {type: Kind} ? T: never;


/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

interface Cat {
  type: 'cat'
  breeds: 'Abyssinian' | 'Shorthair' | 'Curl' | 'Bengal'
}

interface Dog {
  type: 'dog'
  breeds: 'Hound' | 'Brittany' | 'Bulldog' | 'Boxer'
  color: 'brown' | 'white' | 'black'
}

type Animal = Cat | Dog

type cases = [
  Expect<Equal<LookUp<Animal, 'dog'>, Dog>>,
  Expect<Equal<LookUp<Animal, 'cat'>, Cat>>,
]

 

---

One step further:

type LookUp<U extends {[Key in K]: string}, T extends string, K extends string = 'type'> = U extends {[Key in K]: T} ? U : never

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

interface Cat {
  kind: 'cat'
  breeds: 'Abyssinian' | 'Shorthair' | 'Curl' | 'Bengal'
}

interface Dog {
  kind: 'dog'
  breeds: 'Hound' | 'Brittany' | 'Bulldog' | 'Boxer'
  color: 'brown' | 'white' | 'black'
}

type Animal = Cat | Dog

type cases = [
  Expect<Equal<LookUp<Animal, 'dog', 'kind'>, Dog>>,
  Expect<Equal<LookUp<Animal, 'cat', 'kind'>, Cat>>,
]

 

---

One step further;

type LookUpKey<U extends {[Key in K]: string}, T extends string, K extends string = 'type'> = U extends {[Key in K]: T} ? U : never;
type LookUpLike<U extends Record<PropertyKey, any>, T extends Record<PropertyKey, any>> = U extends T ? U : never;

type LookUp<
  U extends Record<PropertyKey, any>, 
  T extends Record<PropertyKey, any> | string, 
  K extends string = 'type'
> = T extends Record<PropertyKey, any> 
      ? LookUpLike<U, T>
      : T extends string
        ? LookUpKey<U, T, K>
        : never;

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

interface Cat {
  kind: 'cat'
  breeds: 'Abyssinian' | 'Shorthair' | 'Curl' | 'Bengal'
}

interface Dog {
  kind: 'dog'
  breeds: 'Hound' | 'Brittany' | 'Bulldog' | 'Boxer'
  color: 'brown' | 'white' | 'black'
}

type Animal = Cat | Dog

type cases = [
  Expect<Equal<LookUp<Animal, 'dog', 'kind'>, Dog>>,
  Expect<Equal<LookUp<Animal, 'cat', 'kind'>, Cat>>,
  Expect<Equal<LookUp<Animal, {kind: 'dog'}>, Dog>>,
  Expect<Equal<LookUp<Animal, {kind: 'cat'}>, Cat>>,
]