Declaration file non-exported types are importable

[mheiber]

TypeScript Version: 3.9.2, 3.8.3, 3.1.6 3.0.3 (and likely more)

Search Terms:

declaration file export, non-exported types, implicit export, dts, .d.ts, es-modules declaration

Code

module.d.ts

export declare const explicitlyExported: {};
declare const implicitlyExported: {};

importer.d.ts

import { implicitlyExported } from "./module"; // no error, is usable import

Expected behavior:

Error: Module '"./module"' has no exported member 'implicitlyExported'.

Actual behavior:

No error.

Even thought it wasn't exported, implicitlyExported is available for import.

Playground Link:

n/a (declaration file issue)

Related Issues:

[mheiber]

As far as we can tell, this is how TS currently decides which mode to use for declaration files. Is this intended? What is the use case for implicit exports / mixed mode?

	has export declaration	no export declaration
has >= 1 import or export statement	ESM-style	implicit exports (mixed mode)
no import or export statements	ESM-style	ambient

[weswigham]

Yeah, this is unfortunately... by design... if you add export {}, you'll get correct private-ness. We could opt to change this, but it'd definitely be a breaking change that walks back something that we've been calling "working as intended" for a long time.

[mheiber]

Thanks @weswigham

[renet]

Are there any plans on changing this behavior, maybe in a major release some time?

[RyanCavanaugh]

No