Allow `export type Foo;` as valid syntax

Search Terms

Exporting a type defined elsewhere. Export a defined type. Type alias circularly references itself. Can't export a type with the same name.

Suggestion

Currently you can only export types right when you declare them:

export type Foo = string | number;

But many people often use ambient files to make their types available globally throughout the project without the need to import them. This however means that the types cannot be exported directly where they define them because the file would cease to be ambient. The current way of coping with this is naming the type differently for internal use and then reexporting it with a different name:

// in types.ts
type _Foo = string | number;

// in index.ts
export type Foo = _Foo;

I suggest that this inconvenience could be easily solved by allowing this syntax:

export type Foo;

Use Cases

As described above, it's useful to export types defined in ambient files.

Examples

As above

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.

No files, tests, or entry points are named. Start by tracing how TypeScript parses and checks export type declarations, then determine the required parser, type-checking, and regression-test changes; done means the proposed syntax is accepted for ambient types without changing JavaScript output.