Type Lookup

by Anthony Fu @antfu

Sometimes, you may want to look up a type in a union by its 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`