Union to Type

index.ts

type UnionToType<
  U extends Record<string, unknown>
> = {
  [K in (U extends unknown ? keyof U : never)]: U extends unknown 
    ? K extends keyof U ? U[K] : never 
    : never
}

type A = {
  a: string
}

type B = {
  b: number
}

type C = {
    c: boolean
}

type D = {
  c: A
}

type Combined = UnionToType<A | B | C | D>

/*
type Combined = {
  a: string;
  b: number;
  c: boolean | A;
}
*/

const combined1: Combined = {
  a: 'test',
  b: 2,
  c: {
    a: ''
  }
}

const combined2: Combined = {
  a: 'test',
  b: 2,
  c: false
}