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Bottom line: everyone who knows coffeescript knows javascript. The inverse is not true. Personal preference, de facto standards, "it's just like ruby" - none of this has to even come into the equation.
>everyone who knows coffeescript knows javascript. Is this necessarily true? You can't put coffeescript between script tags and just have it run, as it's designed to use incompatible idioms which need to be 'compiled' to js (because js is ugly and awkward). It's entirely possible someone who has only ever used coffeescript and has a deep aversion to js has no idea how to actually write the js version of whatever it i…
That's assuming they've never had to debug one of their programs, ever. Even with a distaste for braces and semi colons they will have to eventually look through the compiled code, or run through something like js2coffee.