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Linking in Local Packages

It can often be useful to try out changes to the packages in the main Backstage repo within your own app. For example if you want to make modifications to @backstage/core-plugin-api and try them out in your app.

To link in external packages, add them to your package.json and lerna.json workspace paths. These can be either relative or absolute paths with or without globs. For example:

"packages": [
"../backstage/packages/core-plugin-api", // New path added to work on @backstage/core-plugin-api

Then reinstall packages to make yarn set up symlinks:

yarn install

With this in place you can now modify the @backstage/core-plugin-api package within the main repo, and have those changes be reflected and tested in your app. Simply run your app using yarn dev (or yarn start for just frontend) as normal.

Note that for backend packages you need to make sure that linked packages are not dependencies of any non-linked package. If you for example want to work on @backstage/backend-common, you need to also link in other backend plugins and packages that depend on @backstage/backend-common, or temporarily disable those plugins in your backend. This is because the transformation of backend module tree stops whenever a non-local package is encountered, and from that point node will require packages directly for that entire module subtree.

Type checking can also have issues when linking in external packages, since the linked in packages will use the types in the external project and dependency version mismatches between the two projects may cause errors. To fix any of those errors you need to sync versions of the dependencies in the two projects. A simple way to do this can be to copy over yarn.lock from the external project and run yarn install, although this is quite intrusive and can cause other issues in existing projects, so use this method with care. It can often be best to simply ignore the type errors, as app serving will work just fine anyway.

Another issue with type checking is that the incremental type cache doesn't invalidate correctly for the linked in packages, causing type checking to not reflect changes made to types. You can work around this by either setting compilerOptions.incremental = false in tsconfig.json, or by deleting the types cache folder dist-types before running yarn tsc.