CODEWITHSUNDEEP
typescript
Matt McCutchen
Matt McCutchen

Reputation: 30929

Intentional that `export {}` shuts off automatic export of all symbols from a TypeScript module declaration?

In a TypeScript external module declaration (either an entire .d.ts file or a declare module block in another file), by default, all symbols are exported even if they are not marked export. Luke Pighetti discovered that adding an export { ... } statement suppresses this behavior, so that only symbols marked export are exported.

Example my-module.d.ts:

export const a = 1;
declare const b = 2;
export {};

consumer.ts:

import { a } from "./my-module";  // OK
import { b } from "./my-module";  // Error

This behavior is potentially useful. Is it intentional, or a TypeScript bug that I should report and not encourage people to rely on?

Upvotes: 3

Views: 410

Answers (2)

Matt McCutchen
Matt McCutchen

Reputation: 30929

andy-ms says the behavior is intentional. (But it's undocumented AFAIK, like so many advanced TypeScript behaviors. :( )

I tracked down the compiler code that is involved. The full list of constructs that shut off automatic export:

  • export { ... }
  • export { ... } from "module"
  • export * from "module"
  • export = ...
  • export default of an expression, but not export default of a function, class, or interface definition

Upvotes: 6

unional
unional

Reputation: 15589

This is intentional.

A TypeScript file can be either a script file or a module file. The differentiation between the to is that module file has at least one top-level import/export.

When a file is a module file, only the code explicitly exported with the export keyword would be available.

For a script file, it is treated as if it is part of the global, meaning your content will be available in other files.

For typings file (*.d.ts), in DefinitelyTyped they are written as script file with the format of declare module x { ... } to define the typings for a specific module.

It is written that way in DefinitelyTyped because it is a repository of many modules, and the tooling needs to know which module it is defined for.

If you create a typings file for a JavaScript library and distribute it with that library, it will be written as a module file(s).

Upvotes: 0

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