I think juniors are often better off working on their own projects if possible, rather than integrating into a larger project. Ideally these should be small and comparatively simple, and they can work their way up to larger projects. Code reviews would allow regular feedback on code quality and allow for potentially problematic code to be identified and refactored away.
I'd only feel comfortable bringing a junior onto a large existing project in cases where I already had good test coverage and continuous integration.
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