While my example isn't an issue in ES6, most of the controversial came from CoffeeScript. Using classes comes with classical inheritance expectations. In Coffee

While my example isn't an issue in ES6, most of the controversial came from CoffeeScript. Using classes comes with classical inheritance expectations. In CoffeeScript, changing class variables would carry through all instances, and that is unexpected from a classical inheritance point of view. However, it made complete sense in the compiled JS.