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

來自 @angular/common:

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

這是 @angular/common 里標準的 api：

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

然后單擊進去之后，看不到具體的實現代碼：

@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

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 文件里。

TypeScript 幫助文檔里對 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，用于聲明一個變量，其原始定義可能并不是來自一個 TypeScript 文件。

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.

報錯：uncaught referenceError: myLibrary is not defined

更多Jerry的原創文章，盡在：“汪子熙”:

總結

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