154

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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2 Answers 2

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196

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.

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4 Comments

Thanks for the information, I assume that the decision was as you say to avoid having to check the context of the keyword, which is a shame as it feels like its something that will trip a few people up and really has no logical difference in behaviour to how you expect public to act, just makes their implementaiton easier.
It is necessary if you are using modules. If you app is toward the larger side, modules are typically a better choice than creating big bundles / loading all your files up front.
@Fenton Didn't you mean "you could argue that they should have used 'public' instead of 'export' ?
@AlanEvangelista you could certainly argue it that way round as well, especially if your background was Java / C# rather than JavaScript.
54

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
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7 Comments

thanks for the additional information, I completely agree with the public/private being at member level, I just found it odd not to be sufficient for all levels of access. However this is a personal opinion and a keyword is a keyword, just wanted to find out more.
I am confused by the first bullet point. Who is suggesting that export should have a third meaning?
I agree with some of what you're saying but I disagree with the statement "There isn't a suitable antonym to export" - "import" is the antonym, and is what TypeScript uses, when you define an exportable class, you then import it into other files export class User { name: string } Another file: import {User} from ""./the_file_path_to_the_user_class; see section 3.3 of the nativescript docs here docs.nativescript.org/angular/tutorial/ng-chapter-3
How would using import to indicate "this value is not exported" be a suitable use of the keyword?
"There isn't a suitable antonym to export (unexport??)" - a suitable antonym for export should be embargo.
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