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window-ref.ts 里的 isBrowser API，封装的是 Angular 规范 API，isPlatformBrowser:
来自 @angular/common:
isBrowser(): boolean {// TODO(#11133): Remove condition when platformId will be always provided
return this.platformId
? isPlatformBrowser(this.platformId)
: typeof window !== 'undefined';
}这是 @angular/common 里规范的 api：
https://angular.io/api/common…
而后单击进去之后，看不到具体的实现代码：
@publicApi
export declare function, 其用法解释：
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 提醒了 Action 的理论实现肯定在另一个地位，很可能是某个 .js 文件里。
TypeScript 帮忙文档里对 declare 关键字的解释：
The TypeScript declare keyword is used to declare variables that may not have originated from a TypeScript file.
TypeScript 的 declare 关键字，用于申明一个变量，其原始定义可能并不是来自一个 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.
报错：uncaught referenceError: myLibrary is not defined
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