The biggest advantage I've found is being able to change library code AND all usages of the library in the same commit. Say you've got three projects:
- library
- app-a
- app-b
and you need to make a change to `library` to support some new thing in `app-a`. If you can publish a new version of `library` and update `app-a` to use it, it's really easy to make a change that's incompatible with `app-b`. Even with a comprehensive `library` test suite, it's easy for Hyrum's law to make an appearance and now you've got some unexpected corner case that's depended on.
With a monorepo, you can immediately see `app-b`'s tests failing and either fix the usage, or re-think your `library` changes.
- library - app-a - app-b
and you need to make a change to `library` to support some new thing in `app-a`. If you can publish a new version of `library` and update `app-a` to use it, it's really easy to make a change that's incompatible with `app-b`. Even with a comprehensive `library` test suite, it's easy for Hyrum's law to make an appearance and now you've got some unexpected corner case that's depended on.
With a monorepo, you can immediately see `app-b`'s tests failing and either fix the usage, or re-think your `library` changes.