Why is my Node.js app using Yarn?

Resolution

Heroku recently added support for Yarn to the Node buildpack - https://devcenter.heroku.com/changelog-items/1065 This checks for the presence of a yarn.lock file in the repo. If you don't want to use Yarn on Heroku, we recommend adding yarn.lock to your .slugignore file https://devcenter.heroku.com/articles/slug-compiler#ignoring-files-with-slugignore