There are project layouts that put implementation files and test files together.

The project organization is a bit weird because we have a mono-repository that contains projects and libraries. The projects are grouped by company and by browser / server / universal.

It’s become increasingly common to divide code into components, rather than by file type. React, for example, allows for the collocation of a components markup and JavaScript. In Svelte, this is taken one logical step further: the Javascript, markup and styling for a component can all exist together in a single `.svelte`​ file

This file that really does nothing but manage other project files is often called a “solution” in some environments.