Should TypeScript Interfaces Be Defined in *.d.ts Files

Question

TypeScript newbie question. In our project, we are using some external JavaScript libraries where we needed to add *.d.ts files. I understand this use case and the reason why we

Answer 1

Another developer suggested we put [the interface] in a *.d.ts file and then we would not need to import it in order to reference it.

Using a .d.ts vs. .ts file is unrelated to providing a type declaration in a local/module or global/script scope.

You can export the interface ErrorFirstCallback, so others will have to import it. Or you don't use export/import to make the file a global script. Then, ErrorFirstCallback can be used without an import. It doesn't matter, if the file has a .ts or .d.ts extension in this case.

When should interfaces that we are defining be defined in a *.d.ts file vs a *.ts file.

What does matter is, that .d.ts files are only seen as compiler input and not emitted to your dist/build folder.

As a good rule of thumb, you put the type in a .ts, if you want to provide it as part of an npm package or public typed API to make your life easier with the build step (it will be emitted as compiler output).

You can use .d.ts files for internally used types of your project.