Why are my Node.js app modules "not found" when booting the app?

Resolution

Heroku doesn't install devDependencies by default as it is considered a production environment https://devcenter.heroku.com/articles/nodejs-support#devdependencies

You can either move any required packages to the dependencies section of package.json, or you can choose to install devDependencies during the build by running:

heroku config:set NPM_CONFIG_PRODUCTION=false