Why does Typescript use the keyword “export” to make classes and interfaces public?

原文： https://stackoverflow.com/questions/15760462/why-does-typescript-use-the-keyword-export-to-make-classes-and-interfaces-publ

Question:

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{..}
}

So now I can use the above code like this:

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

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?

This would give resulting code like:

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

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Answers:

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.

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.

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

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