Once upon a time a group of talented engineers gathered and built a wonderful project with AngularJS. The project grew and expanded. At some point, the project became very hard for maintenance and development. A group of talented engineers decided to rewrite everything to a new language, which wasn't known to anyone from a group. Why we chose Elm? How it differs from JS? How we rewrote 50k LoC into new language within a year? Is it easier now to develop new features? Do we regret the choice?