Install devDependencies on Heroku

Heroku by default installs only the production dependencies, ignoring the development dependencies under devDependencies.

Setting the npm production variable to false do the trick:

heroku config:set NPM_CONFIG_PRODUCTION=false

More info are available at the Heroku Node.js Support page.

Keeping NPM_CONFIG_PRODUCTION true, I used Heroku's script hooks:

"scripts": {
  ...
  "heroku-prebuild": "export NPM_CONFIG_PRODUCTION=false; export NODE_ENV=; NPM_CONFIG_PRODUCTION=false NODE_ENV=development npm install --only=dev --dev",
  "heroku-postbuild": "export NPM_CONFIG_PRODUCTION=true; export NODE_ENV=production;",
   ...
},

(Finally) worked for me.