export default type

[hopeless-programmer-online]

Search Terms

export default type

Suggestion

Allow export default for type.

Use Cases

It would be very nice to have export default type to match with other default exports such as classes and interfaces.

Right now it looks very inconsistent to be able to export anything as default and only types with named export:

export default class MyClass {}
export default interface MyInterface {}
export type MyType = {}

The only way to achieve this now is to use annoying workaround:

type X = number | string

export { X as default }

This may cause confusion for newbies as there are no clear reasons why classes and interfaces can be exported as default, but not the types.

Examples

// x.ts
export default type X = {}

// main.ts
import X from './x'

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

(not sure on the last)

[harrysolovay]

I often want to export values which double as types. For instance.

export const Tag = Symbol.for("@my-app/entry.Tag");
export type Tag = typeof Tag;

It feels to me like this should be possible with default exports as well.

[hopeless-programmer-online]

Any updates on this?

[PolariTOON]

TypeScript 5.0 (in #52203) introduced a new option called --verbatimModuleSyntax that makes the following code produce the following error: An 'export default' must reference a value when 'verbatimModuleSyntax' is enabled, but 'X' only refers to a type., because the default export X is not clearly marked as a type:

type X = number | string;
export default X;

And a workaround is to mark X as a type in the export declaration by using a named export:

type X = number | string;
export type {X as default}; // or export {type X as default};

But that looks fairly indirect. Having export default type X; would also help making that more straightforward.

[AndreiSoroka]

was:

type SomePayload = {
 data1: number;
 data2: string;
};
export default SomePayload;

import type SomePayload from './SomePayload.type'

now:

export type SomePayload = {
 data1: number;
 data2: string;
};

import type { SomePayload } from './SomePayload.type'

seems like a bug, because I have 1 type in 1 file

[movahhedi]

For those who were looking for "Type annotations for export default", follow #13626.