Fun things to do with Typescript 2.8

gistfile1.txt

type Union =
  { type: 'A', a: string } |
  { type: 'B', b: number }

type FilterType<T, K> = (T extends { type: K } ? T : never);
type RemoveType<T, K> = (T extends K ? never : T);

type MappedUnion = {[P in Union['type']]: RemoveTypeFromRecord<FilterType<Union, P>, 'type'>};

type MappedUnionTarget = {
  A: {  a: string }
  B: {  b: number }
}

type RemoveType<T, K> = (T extends K ? never : T);
type RemoveTypeFromRecord<T, K> = {[L in RemoveType<keyof T, K>]: T[L]}
type Test = RemoveTypeFromRecord<{A: string, B: number, C: boolean[]}, 'C'>;
type TestTarget = {A: string, B: number};

function cvt3(x : Test): TestTarget {
  return x;
}

function cvt4(x : TestTarget): Test {
  return x;
}

// const z: RemoveTypeFromRecord<{type: 'A', a: string}, 'type'> = {'a': 'tadf'};
// type Blerp = {[K in keyof MappedUnionTarget]: RemoveType<MappedUnionTarget[K]>};


function cvt1(x: MappedUnion): MappedUnionTarget {
  return x;
}

function cvt2(x: MappedUnionTarget): MappedUnion {
  return x;
}

const v: MappedUnion = {
  'A': { a: "d"},
  'B': { b: 3},
}

Related to https://stackoverflow.com/questions/46745785/transform-union-into-a-map/49263466