Pyret is implemented in JavaScript so we can have a completely in-browser runtime. And since the Pyret compiler is implemented in Pyret, that comes along for free and can be run in the browser as well. JS is far from an ideal compilation target (especially for a functional language, it requires jumping through some extra hoops [1]), but compiling to it lets us deliver the language to students with no installation necessary [2].

[1] http://www.ustream.tv/recorded/43777177 [2] https://code.pyret.org