Type a function based on its argument value.

  const helpMeTypeThis = (obj: MyObj, bool = false)
    : {id?: string | undefinded, [key: string]: any }[] => {
    return obj.map(...)
  }

helpMeTypeThis(obj)  // => { [key: string]: any }
helpMeTypeThis(obj, false)  // => { [key: string]: any }
helpMeTypeThis(obj, true)  // => { id: string, [key: string]: any }