Exponent uses a monolithic code repository (rather than many small code repos) to manage our many interdependent projects. Our repo is actually setup very similarly to Facebook's mono-repo, though obviously considerably smaller in size. We were encountering _many_ of the same problems as Facebook with our NPM setup -- long CI/CD build times, indeterminate node_modules directories that caused our projects and tools to work for some people on our team but not for others, and the inability to do "offline-only" npm installs in CI.
We actually talked with our friends at Facebook about these problems, and tried many of the same approaches they did -- shrinkwrap everything, checking in node_modules, uploading the node_modules folder to separate place (we used another completely separate Git repo), etc. All these approaches either didn't work well or were difficult to maintain, especially on a small team.
Yarn has fixed all these issues for us.
One not-yet-super-publicized feature of Yarn (though I'm told there is a blog post coming) that has been super useful at Exponent is the ability to build a distributable offline cache of dependencies. We have a "node_modules-tarballs" folder in our mono-repo that contains tarballs of all the dependencies for every single project that we maintain at Exponent. Yarn will pull from this cache first before fetching from the NPM registry. This offers HYPER-predictability...it's a level of confidence over and above the "yarn.lock" file, and let's me sleep well at night knowing that my team can get up and running quickly no matter where they're working, how reliable their internet connection is, etc. No more worrying about "did I forget to update the shrinkwrap" -- things just work.
As JS developers, we all, beginners and experts alike, can experience this "JavaScript fatigue" that we always hear about. Fighting with tools is never fun. We just want to write code!
As we've started to use Yarn at Exponent, both on our engineer's machines as well as in our continuous integration and deployment environments, we've at least had one less tool to fight with. Yarn works, works fast, uses the great parts of the NPM ecosystem that we know and love, and most importantly, is _predictable_.