为什么 Typescript 使用关键字“export”来公开类和接口？
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Why does Typescript use the keyword "export" to make classes and interfaces public?
提问by Grofit
While dabbling with Typescript I realised my classes within modules (used as namespaces) were not available to other classes unless I wrote the exportkeyword before them, such as:
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export在它们之前写了关键字，例如：
module some.namespace.here
{
export class SomeClass{..}
}
So now I can use the above code like this:
所以现在我可以像这样使用上面的代码：
var someVar = new some.namespace.here.SomeClass();
However I was just wondering why this keyword is used opposed to just using the publickeyword 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?
但是，我只是想知道为什么使用此关键字而不是仅使用public在方法级别使用的关键字来表示方法或属性应可从外部访问。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？
This would give resulting code like:
这将给出如下结果代码：
module some.namespace.here
{
public class SomeClass{..}
}
回答by Fenton
The primary reason is that exportmatches 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.
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来解释这一点.
In TypeScript, marking a class member as publicor privatehas 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.
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，您可以使用它来阻止您的 TypeScript 代码访问它不应该访问的内容。
With the exportkeyword, the JavaScript adds a line to add the exported item to the module. In your example: here.SomeClass = SomeClass;.
使用export关键字，JavaScript 添加一行以将导出的项目添加到模块中。在您的例子：here.SomeClass = SomeClass;。
So conceptually, visibility as controlled by publicand privateis just for tooling, whereas the exportkeyword changes the output.
所以从概念上讲，由publicand控制的可见性private仅用于工具，而export关键字更改输出。
回答by Ryan Cavanaugh
A few things to add to Steve Fenton's answer:
Steve Fenton 的回答要补充几点：
exportalreadymeans two different things (depending on whether it's at top-level or not); making it mean a third is probably worse than addingpublic/private- It's definitely not to make the implementation easier; the added complexity of
publicvsexportis trivial. We've changed keywords around a bunch already; it's not difficult. - The default visibility of class members mustbe public to align with the ES6 class proposal, therefore we need some keyword to indicate "not public". There isn't a suitable antonym to
export(unexport??), soprivateis the logical choice. Once you haveprivate, it would be somewhat insane to not choosepublicas its counterpart - Use of
exportto modify visibility in internal modules is the best-guess alignment with ES6 modules
export已经意味着两个不同的东西（取决于它是否在顶级）；这意味着三分之一可能比添加public/private- 绝对不是为了让实现更容易；
publicvs增加的复杂性export是微不足道的。我们已经改变了一堆关键字；这并不难。 - 类成员的默认可见性必须是 public 以与 ES6 类提案保持一致，因此我们需要一些关键字来表示“不公开”。
export(unexport??)没有合适的反义词，所以private是合乎逻辑的选择。一旦你有了private，不选择public作为它的对手会有点疯狂 - 使用 of
export来修改内部模块中的可见性是与 ES6 模块的最佳匹配
