Angular8: Date对象的原型继承

Question

我试图找到一种方法在Angular(Typescript)中添加一些方法到Date prototype中，并通过GitHub找到了下面的解决方案，效果很好。

date.extensions.ts

export {}

// DATE EXTENSIONS
// ================

declare global {
   interface Date {
      addDays(days: number, useThis?: boolean): Date;
      isToday(): boolean;
      clone(): Date;
      isAnotherMonth(date: Date): boolean;
      isWeekend(): boolean;
      isSameDate(date: Date): boolean;
   }
}

Date.prototype.addDays = (days: number): Date => {
   if (!days) return this;
   console.log(this);
   let date = this;
   date.setDate(date.getDate() + days);

   return date;
};

Date.prototype.isToday = (): boolean => {
   let today = new Date();
   return this.isSameDate(today);
};

Date.prototype.clone = (): Date => {
   return new Date(+this);
};

Date.prototype.isAnotherMonth = (date: Date): boolean => {
   return date && this.getMonth() !== date.getMonth();
};

Date.prototype.isWeekend = (): boolean => {
   return this.getDay() === 0 || this.getDay() === 6;
};

Date.prototype.isSameDate = (date: Date): boolean => {
   return date && this.getFullYear() === date.getFullYear() && this.getMonth() === date.getMonth() && this.getDate() === date.getDate();
};

参考：https://github.com/Microsoft/TypeScript/issues/7726#issuecomment-234469961

问:谁能告诉我为什么export {}会写在TS文件的开头，为什么有必要在这里添加？

Answer 1

如果不从date.extensions.ts中导出任何内容，该文件将不会考虑模块，并且会出现三个错误

error TS2669: Augmentations for the global scope can only be directly nested in external modules or ambient module declarations.

error TS2306: File 'date.extensions.ts' is not a module.

and an error about the Date type 

只需使用export {}将文件作为模块使用，因为它已经导出了一些东西