Why is the Node buildpack only installing some of my dependencies?

Issue

When I go to build my application on Heroku, the install step is not installing all of my dependencies.

For example, my Heroku logs will output:

Installing node modules
added 60 packages in 3.918s

However, I should have more node modules being installed.

Resolution

The package-lock.json for your app is out of date. Locally, run npm install and make sure the changes to the package-lock.json are checked into git and made available to Heroku.