It might be on a case to case basis, but for me C++ and Emscripten was an almost drop in replacement. My survey compiler uses ncurses, wxWidget/gtk and web tool

It might be on a case to case basis, but for me C++ and Emscripten was an almost drop in replacement. My survey compiler uses ncurses, wxWidget/gtk and web toolkit for native screen display. I read the emscripten documentation that you could call pure javascript and pass data back from javascript too. I wondered if you could use Dojo as a widget library replacement for native toolkits like wxWidgets etc. I tried it out and was able to get things working in 2-3 days. And I'm not good at Javascript and have not really tried Dojo before either. I was able to get Sencha to work, but not Sencha Touch. Dojo mobile is working fine as well (but I have to tweak it). The novelty is not asm.js but using the browser as a Windowing System and a JavaScript library as your UI library. The survey compiler is open source and if you are interested in getting started I and point you to the relevant sources. What Emscripten enables me is open up the tablet space for surveys with hardly any effort using toolkits like Phonegap.