Why is my Node.js build failing because of an outdated Yarn lockfile?

Resolution

This issue commonly occurs when your application uses Yarn but some other tool modifies the package.json file without invoking yarn install. An example would be using npm to install a new module instead of Yarn, or manually updating a version requirement by hand.

To resolve this, run yarn install and check in the updated yarn.lock file.

$ yarn install
$ git add yarn.lock
$ git commit -m "Updated Yarn lockfile"
$ git push heroku master