export type WithAge<TSource> = TSource & {
  age: number
}

export function isWithAge(obj: any): obj is WithAge<{}> {
  return typeof obj.age === 'number';
}

export function extendWithAge<
  TSource
>(
  sourceObj: TSource,
  age: number,
): WithAge<TSource> {
  const extension = { age };
  return { ...sourceObj, ...extension };
}