Module augmentation: TypeScript overrides declarations instead of merging them

[mikeconley12]

TypeScript Version: 4.1.0-dev.20201031

Search Terms: "declare module", "module augmentation"

Code

codesandbox

Expected behavior: TypeScript should merge two separate declarations of the ParsedPath interface.

Actual behavior: TypeScript overrides the declaration of the ParsedPath interface. This behavior can be fixed by uncommenting the line export type Foo = ""; in types.ts file.

Playground Link: codesandbox

Related Issues:

[MartinJohns]

I'm not too sure about the internal details, but you see this behavior because types.ts is not a module. You can only augment modules within modules or declaration files. Once you add an import or export statement it's a module. Alternatively you can make types.ts a decaration file by renaming it to types.d.ts.

[mikeconley12]

Thanks for the explanation!