How to define types for local JS file

[sonovice]

I am having troubles using a simple one-file JS library with this template. This is my folder structure:

src/app.ts
src/verovio.js <-- the library
src/types/verovio.d.ts

The app.ts consists of the following:

/// <reference path="./types/verovio.d.ts" />
import * as verovio from './verovio'

And verovio.d.ts:

declare module 'verovio' // Todo: Dirty, do it properly!

I've also put src/types into the typeRoots property of my tsconfig.json (which shouldn't be necessary anymore). But I still get the same error over and over again:

error TS7016: Could not find a declaration file for module './verovio'. '/foo/bar/project/src/verovio.js' implicitly has an 'any' type.

Any ideas what I'm doing wrong? This is at least how I got it to work in another project.