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

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{..}
}

Answer 1

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.

Answer 2

A few things to add to Steve Fenton's answer:

• export already means two different things (depending on whether it's at top-level or not); making it mean a third is probably worse than adding public/private
• It's definitely not to make the implementation easier; the added complexity of public vs export is trivial. We've changed keywords around a bunch already; it's not difficult.
• The default visibility of class members must be 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??), so private is the logical choice. Once you have private, it would be somewhat insane to not choose public as its counterpart
• Use of export to modify visibility in internal modules is the best-guess alignment with ES6 modules