为什么 Typescript 使用关键字“export"?使类和接口公开? [英] Why does Typescript use the keyword "export" to make classes and interfaces public?

问题描述

在涉足 Typescript 时，我意识到模块中的类(用作命名空间)对其他类不可用，除非我在它们之前写了 export 关键字，例如:

While dabbling with Typescript I realised my classes within modules (used as namespaces) were not available to other classes unless I wrote the export keyword before them, such as:

module some.namespace.here
{
   export class SomeClass{..}
}

所以现在我可以像这样使用上面的代码:

So now I can use the above code like this:

var someVar = new some.namespace.here.SomeClass();

然而，我只是想知道为什么使用这个关键字而不是仅仅使用 public 关键字，后者在方法级别使用来表示方法或属性应该可以从外部访问.那么为什么不使用相同的机制来使类和接口等在外部可见呢?

However I was just wondering why this keyword is used opposed to just using the public keyword which is used at method level to signify that a method or property should be externally accessible. So why not just use this same mechanism to make classes and interfaces etc externally visible?

这将给出如下结果代码:

This would give resulting code like:

module some.namespace.here
{
   public class SomeClass{..}
}

推荐答案

主要原因是 export 匹配 ECMAScript 的计划.您可能会争辩说他们应该使用export"而不是public"，但除了export/private/protected"是一组匹配不佳的访问修饰符之外，我相信两者之间存在细微的差异，可以解释这一点.

The primary reason is that export matches the plans for ECMAScript. You could argue that "they should have used "export" instead of "public", but asides from "export/private/protected" being a poorly matched set of access modifiers, I believe there is a subtle difference between the two that explains this.

在 TypeScript 中，将类成员标记为 public 或 private 对生成的 JavaScript 没有影响.它只是一个设计/编译时工具，可用于阻止 TypeScript 代码访问不该访问的内容.

In TypeScript, marking a class member as public or private has no effect on the generated JavaScript. It is simply a design / compile time tool that you can use to stop your TypeScript code accessing things it shouldn't.

使用 export 关键字，JavaScript 添加一行以将导出的项目添加到模块中.在您的示例中:here.SomeClass = SomeClass;.

With the export keyword, the JavaScript adds a line to add the exported item to the module. In your example: here.SomeClass = SomeClass;.

因此从概念上讲，由 public 和 private 控制的可见性仅用于工具，而 export 关键字会更改输出.

So conceptually, visibility as controlled by public and private is just for tooling, whereas the export keyword changes the output.

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