verbatimModuleSyntax and exported const enum

[magic-akari]

Bug Report

verbatimModuleSyntax will be introduced in [email protected] and importsNotUsedAsValues and preserveValueImports will be deprecated.
But there is a const enum export issue that has not been fixed.

🔎 Search Terms

• verbatimModuleSyntax
• export const enum
• importsNotUsedAsValues
• preserveValueImports

🕗 Version & Regression Information

[email protected]

⏯ Playground Link

Playground link with relevant code

Playground multi-file mode is broken.
Check this gist

💻 Code

// @filename: foo.ts
export const enum Foo {
  x = 1,
}

// @filename: main.ts
import { Foo } from "./foo.js";
// You cannot write something like the following
// import type { Foo } from "./foo.js";

console.log(Foo.x);

🙁 Actual behavior

SyntaxError: The requested module './foo.js' does not provide an export named 'Foo'

🙂 Expected behavior

TSC Report Error: const enum cannot be exported in verbatimModuleSyntax mode.

🔗 related issue

• Design Meeting Notes, 1/25/2023 #52469

[DanielRosenwasser]

I believe this is by-design unless you run with preserveConstEnums. CC @andrewbranch

[andrewbranch]

verbatimModuleSyntax is supposed to be a superset of isolatedModules which is supposed to imply preserveConstEnums, IIRC.

[DanielRosenwasser]

I think it just errors when preserveConstEnums isn't enabled - I don't think it actually enables it. But I could be wrong!

[andrewbranch]

isolatedModules does imply preserveConstEnum, but there were many places in the codebase where I forgot to replace an options.isolatedModules with a check for isolatedModules or verbatimModuleSyntax.

[mrazauskas]

Would it be possible to mentioned in the documentation that verbatimModuleSyntax: true does influence const enums?

I noticed that const enums are not inlined at use sites with verbatimModuleSyntax: true. Looks like the behaviour is similar to isolatedModules: true, but it can be I missed something.

EDIT Perhaps isolatedModules should be replaced with verbatimModuleSyntax in all places there documentation speaks about const enums? Simply because isolatedModules got deprecated and replaced with verbatimModuleSyntax? Or that is not exactly correct?

[andrewbranch]

isolatedModules isn’t deprecated 🤔

[mrazauskas]

Ups.. I got mixed up. Despite of that, it would be useful to mention in documentation how verbatimModuleSyntax works with const enum.

[andrewbranch]

I’m not totally convinced that isolatedModules should actually mess with const enum inlining. It’s supposed to be simply a set of checks that code can be transpiled with only single-file info, but somewhere along the way I think someone got mixed up and decided it should emit as if it lacks whole-program info. But that principle isn’t consistently applied, and I don’t see why it would be desirable anyway.

[mrazauskas]

Are you trying to say that this is possibly a bug? I mean, it felt like const enums should be inlined with verbatimModuleSyntax: true, but it can be I missed some detail.

Would it make sense to open a separate issue? I can do. Just didn’t want to create unnecessary noise.

[andrewbranch]

I’m catching up here: #16671

[curiousdannii]

I had to add verbatimModuleSyntax to use Svelte, and now my const enums aren't optimised. This is quite unfortunate, and also seemingly undocumented.