grunt-contrib-cssmin | - Compress CSS files | Compression library

Node.js 4.4.4 is absolutely ancient.

It doesn't even show up on this page of releases, which says in part:

After six months, odd-numbered releases (9, 11, etc.) become unsupported, and even-numbered releases (10, 12, etc.) move to Active LTS status and are ready for general use. LTS release status is "long-term support", which typically guarantees that critical bugs will be fixed for a total of 30 months.

Version 4.4 is well beyond its maintenance window, and I strongly urge you to upgrade this project.

Having said that, there's a good chance you can get Heroku to run Node.js 10 to match your local version. Heroku only officially supports the current and active LTS releases, but it doesn't prevent you from using older releases:

Since Heroku is based on a standard Ubuntu Linux stack, you can run most Node versions (>= 0.10.0) on the platform. However, the testing and support focus of the buildpack will be oriented around active LTS and Stable releases.

Update the engines section in your package.json:

Node.js doesn't get bundled with the source code of apps. The package.json might have a section called "engines" in which it will state what version you should be using.

If the root package.json doesn't have the "engines" section, then it may be posable that the some of the dependencies do say which version they require to be used. It would be kind of annoying going through each one to check, so a good way would be just to download a version of Node and run npm install. If everything works, then you know that the Node version the app was created in is most likely older (its a bit tedious, I know).

Another thing you could look for (but might not be to helpful) would be to check when the files of the source code were created (especially the package.json file), and find the Node version that was released around that time. This wont be as accurate as the first method but it will give you a working version of Node.

When it comes down to it though, its probably always best to use the most up to date version (or the most recent LTS version) as they come with all the latest security patches and improvements.