According to this comment,
Indeed, devDependencies in the root package.json can be used in all packages in the monorepo. This is generally used to co-locate test-related deps and config (jest, eslint, etc).
A couple caveats:
This only works for non-CLI devDependencies. If you need to run a CLI from a child package, that CLI needs to be in that package's devDependencies. (transpiling, flowcopy, etc) All dependencies for a given child package still need to be listed in that child's package.json. (obvious, perhaps? just clarifying)
I see that devDependency in the root directory can be used for all packages. However, I can't see an obvious way to install devDependency only in root. Is there a such command or should I simply use npm/yarn -D install to do this? (or manually modify package.json)
If you use Yarn Workspaces, you can hoist shared dependencies in the root node_modules. To install dependencies in the root workspace, use the -W flag.
yarn add jest --dev -W
As you mentioned, these can now be used in any package.