It was a thing for a hot minute in the early 2010s, before ES6, TypeScript, and the React/Angular/Vue trifecta came along and solidified what JavaScript would be from ~2015 onwards.

I ran into Coffeescript code on GitHub for the first time in years last week. Oh, Coffeescript, I remember that used to be a thing.

It was the default for a while for Rails apps and Wiki tells me GitHub & Dropbox adopted it before moving to Typescript. I think for Rails shops it was popular and there was a brief time where it looked like it might become the thing to do on the front end. But I don't think it ever got the interop with existing JS right.

Ultimately, improvements to JS came along and took the wind out of CoffeeScript's sails. Then TypeScript came along and killed whatever interest remained.