window-ref.ts 裡的 isBrowser API，封裝的是 Angular 標準 API，isPlatformBrowser:

![](https://img-blog.csdnimg.cn/img_convert/8c0bdb14da1097e3e761bb1cb9c5e191.png)

來自 @angular/common:

![](https://img-blog.csdnimg.cn/img_convert/e98a833de425e0a9872fc88575692428.png)

```typescript
 isBrowser(): boolean {
    // TODO(#11133): Remove condition when platformId will be always provided
    return this.platformId
      ? isPlatformBrowser(this.platformId)
      : typeof window !== 'undefined';
  }
```

這是 @angular/common 裡標準的 api：

https://angular.io/api/common/isPlatformBrowser

![](https://img-blog.csdnimg.cn/img_convert/a9b1b22e0380da5e62e1a373a57b2940.png)

然後單擊進去之後，看不到具體的實現程式碼：

@publicApi

export declare function, 其用法解釋：

> Declare vs. var

var creates a new variable. declare is used to tell TypeScript that the variable has been created elsewhere. If you use declare, nothing is added to the JavaScript that is generated - it is simply a hint to the compiler.

For example, if you use an external script that defines var externalModule, you would use declare var externalModule to hint to the TypeScript compiler that externalModule has already been set up

```typescript
export declare class Action{
...
}
```

The class's real implementation is probably in somewhere else - maybe a .js file.

declare 提示了 Action 的實際實現一定在另一個位置，很可能是某個 .js 檔案裡。

![](https://img-blog.csdnimg.cn/img_convert/db9f4849c5f22d207ba43c46f945b4c5.png)

![](https://img-blog.csdnimg.cn/img_convert/2c1e867ceeb00079b914d23a00db0638.png)

TypeScript 幫助文件裡對 declare 關鍵字的解釋：

> The TypeScript declare keyword is used to declare variables that may not have originated from a TypeScript file.

TypeScript 的 declare 關鍵字，用於宣告一個變數，其原始定義可能並不是來自一個 TypeScript 檔案。

> For example, lets imagine that we have a library called myLibrary that doesn’t have a TypeScript declaration file and have a namespace called myLibrary in the global namespace. If you want to use that library in your TypeScript code, you can use the following code:

declare var myLibrary;

The type that the TypeScript runtime will give to myLibrary variable is the any type. The problem here is that you won’t have Intellisense for that variable in design time but you will be able to use the library in your code.

Another option to have the same behavior without using the declare keyword is just using a variable with the any type:

var myLibrary: any;
Both of the code examples will result in the same JavaScript output but the declare example is more readable and expresses an ambient declaration.

![](https://img-blog.csdnimg.cn/img_convert/48c0ed79cc1298141edb4cf2ebea5ad9.png)

報錯：uncaught referenceError: myLibrary is not defined

![](https://img-blog.csdnimg.cn/img_convert/8ffc2db7e7db05562827e0f6a7cc8908.png)
![](https://img-blog.csdnimg.cn/img_convert/de331d28785833ca4a3ce0b63427b27d.png)

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