如何从TypeScript接口的'data'键属性中提取属性？

Question

我有以下代码：

export interface ISubjectData {
  goal: IGoal,
  subjects: ISubject[]
}

export interface IChapterData {
  subject: ISubject
  chapters: IChapter[]
}

export interface ISubjectChapterData {
  type: string,
  data: ISubjectData | IChapterData
}

如何从以上界面中提取data.subjects？