When I joined Box in 2013, the web application front-end was in a typical state for a fast-growing company - what had started out as a small codebase maintained by a few engineers had grown into a massive codebase maintained by dozens of engineers. This meant that new features were built on top of old ones and engineers feared making changes to the code because no one was sure which features the changes would affect. There were tests to help with this, but they were unreliable due to the tightly-coupled nature of the code.