Extension authors can write most of their application in JS. However, they may still need the ability to interact with native code. This is important so that they can use * precompiled external libraries (imagemagick) * perform operations in native code which would be too slow in JS Both of these use-cases can and should be solved with a FFI library which is exposed to JavaScript. See mfinkle's post about an early prototype based on python ctypes. Solutions such as SWIG may also be useful for scripting more complex C++ APIs.
