Around 2012, I had sunk several years into becoming extremely familiar with JavaScript when I switched onto a team which was using CoffeeScript. I immediately liked it: it addressed some of the things I'd always found irritating about JavaScript, and I found it very easy to pick up. I was not a Ruby or even Python user at that time, so that didn't influence my decision either.

But you're right that you did have to know enough JS to understand what CoffeeScript was doing under the hood when things went wrong.