The export default interface is allowed with verbatimModuleSyntax enabled

[ueokande]

🔎 Search Terms

• verbatimModuleSyntax
• export default interface

🕗 Version & Regression Information

• I can reproduce on v5.0.4 and the nightly version.

⏯ Playground Link

https://www.typescriptlang.org/play?target=99&noImplicitReturns=false&alwaysStrict=false&noImplicitThis=false&strictPropertyInitialization=false&strictBindCallApply=false&strictFunctionTypes=false&strictNullChecks=false&noImplicitAny=false&ts=5.5.0-dev.20240224&esModuleInterop=false&declaration=false&jsx=0&verbatimModuleSyntax=true&allowSyntheticDefaultImports=false#code/KYDwDg9gTgLgBAE2AMwIYFcA28CWA7GYKNAY2DgEk4BvAXyA

💻 Code

export default interface I {}

🙁 Actual behavior

The compiler allows export default interface I {} with verbatimModuleSyntax enabled.

🙂 Expected behavior

When we try to export an interface separately with the definition and export default syntax, the tsc outputs an error.

interface I {}
export default I

An 'export default' must reference a value when 'verbatimModuleSyntax' is enabled, but 'I' only refers to a type.

We can export an interface with export default interface I {...} even if the exported name does not reference a value, it references an interface. I have no idea whether the tsc should output an error in that case since I cannot found such case in the test. I think it can either be an error or needs to update the error message.

Additional information about the issue

No response

[whzx5byb]

Related: #41409

Maybe it's just because we don't have the export default type symbol?

[ueokande]

I'm not sure. But by the error message, I guess, the compiler expects that the exported symbol is a value. I think the tsc should output an error if we could use export default type.

[andrewbranch]

This is a bit subtle, but it’s working as designed. The goal with verbatimModuleSyntax is to ensure that all module-related valid JavaScript code gets emitted as-is. export default I is a valid JavaScript statement, so we enforce that it can be emitted as-is without causing errors. On the other hand, export default interface I {} is not valid JavaScript; it’s TypeScript-only code that happens to have export default modifiers on it, which sometimes also appear in valid JavaScript code. It’s allowed for the same reason that export interface I {} is allowed. What matters is what kind of statement the top-level thing is. Even though export default I and export default interface I {} have the same effect, the former is an ECMAScript-defined ExportDeclaration, while the latter is a TypeScript InterfaceDeclaration.

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[ajmas]

It is not clear in that case what the 'correct' way of dealing with this is?

If I follow a 'one interface declaration' by file approach:

interface IMyInterface { 
  // interface props
}

Then none of these work:

export default IMyInterface;
export IMyInterface;
export { IMyInterface };
export default type IMediaItem;

I feel like I am forced to use the verbatimModuleSyntax, simply because there isn't a clear 'correct' solution. At least the TS error 1284, doesn't provide sufficient clues.

BTW If I can track down an answer, I'll provide it here.

[andrewbranch]

export default interface IMyInterface {
  // interface props
}

and

interface IMyInterface {
  // interface props
}
export type { IMyInterface as default };

are two verbatimModuleSyntax-compatible ways of default-exporting an interface.

[rauschma]

Given that this works:

export default interface Intf {}

Shouldn’t either one of the following work too? Why should interfaces get special syntactic treatment?

export default type MyType = {};
export default type SomeOtherType;

[andrewbranch]

I’m guessing because interface is a reserved word in ECMAScript whereas type is not. Both of those constructions would need ASI restrictions:

export default type
MyType = {};

export default type
SomeOtherType;