"Cannot find module" in Node.js at runtime

Issue

Your Node.js app builds successfully but, when it's run, you get a "cannot find module" error like:

Error: Cannot find module 'foobar'

Resolution

First, you should temporarily disable any module caching:

$ heroku config:set NODE_MODULES_CACHE=false

Next, you should ensure that you aren't accidentally checking your modules into git.

Finally, you should check to ensure that your dependencies are correctly listed in package.json.