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

在尝试使用Typescript时，我意识到模块中的类(用作名称空间)对于其他类是不可用的，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{..}
}

所以现在我可以像上面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();

但是，我只是想知道为什么使用此关键字，而不是仅仅使用public关键字，而在方法级别使用public关键字表示应该从外部访问方法或属性.那么，为什么不使用这种相同的机制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?

这将给出结果代码，例如:

This would give resulting code like:

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

推荐答案

主要原因是export与ECMAScript的计划匹配.您可能会争辩说他们应该使用导出"而不是公共"，但是导出/私有/受保护"的辅助功能是一组访问修饰符不匹配的，我认为两者之间存在细微的差异，这可以解释这一点.

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无效.它只是一个设计/编译时工具，您可以使用它来阻止TypeScript代码访问它不应该访问的内容.

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.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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