SO

index.ts

type Elem = Param;

type HandleFalsy<T extends Param> = (
  T['x'] extends undefined
  ? T['y']
  : (
    unknown extends T['x']
    ? T['y'] : T['x']
  )
)

type ArrayMap<
  Arr extends ReadonlyArray<Elem>,
  Result extends any[] = []
  > = Arr extends []
  ? Result
  : Arr extends readonly [infer H, ...infer Tail]
  ? Tail extends ReadonlyArray<Elem>
  ? H extends Elem
  ? ArrayMap<Tail, [...Result, HandleFalsy<H>]>
  : never
  : never
  : never;

type Param<X = unknown, Y = unknown> = {
  x?: X
  y: Y
}

type Json =
  | null
  | string
  | number
  | boolean
  | Array<JSON>
  | {
    [prop: string]: Json
  }


const tuple = <
  X extends Json,
  Y extends Json,
  Tuple extends Param<X, Y>,
  Arr extends Tuple[]
>(data: [...Arr]) =>
  data.map(elem => 'x' in elem ? elem.x : elem.y) as ArrayMap<Arr>

// [42, "only y"] 
const result1 = tuple([{ x: 42, y: 'str' }, { y: 'only y' }])

// [42, "only y", "100"]
const result2 = tuple([{ x: 42, y: 'str' }, { y: 'only y' }, { x: '100', y: 999999 }])