CoffeeScript

Jeremy Ashkenas pushed the first commit on Christmas Eve 2009. The premise was that JavaScript had a good core buried under poor syntax, so CoffeeScript compiled line by line into readable JS: significant indentation instead of braces, -> for functions, comprehensions, classes, and => which bound this correctly years before the language could do it itself.

Ruby on Rails made it the default in 2011 and for a few years it was hard to avoid. Then ES2015 arrived with arrow functions, classes, destructuring and template strings — several of them directly inspired by CoffeeScript — and the argument evaporated. What remains is a stratum of codebases from that period and a clean example of a language winning by making itself unnecessary.