There are several approaches in emscripten to talk to the JS/HTML5 side from the C/C++ side:
1) wrappers for popular native APIs and frameworks to simplify porting existing code: apart from the 'expected' C and C++ runtime wrappers, emscripten also offers wrappers for GL, GLES 1..3, EGL, GLFW, GLU, GLUT, GLEW, SDL and OpenAL, these generally translate (and sometimes emulate) the behaviour of the native APIs to the underlying HTML5 APIs (e.g. GL to WebGL, SDL Audio and OpenAL to WebAudio, etc...)
2) there are also smaller emscripten-specific APIs that are essentially minimal shims around HTML5 APIs, for instance to initialize a WebGL context, one could either use the EGL or SDL wrappers which add some code bloat and don't directly map to WebGL initialization attributes, or one can use the emscripten_webgl_*() functions which map directly to the WebGL context initialization process, there are also similar small shims for input, XMLHttpRequest, WebWorkers and others, the advantage of these direct HTML5 shims is that they introduce less code bloat, but they don't exist anywhere outside the emscripten platform
3) emscripten also provides a number of different ways to easily write your own shims to call from C/C++ into JS, and the other way around, even with marshalling complex C++ data types (see: https://kripken.github.io/emscripten-site/docs/porting/conne...)
I really like that emscripten is much more than just a compiler toolchain with C/C++ runtime, there has been a tremendous amount of work to make it as simple as possible to port existing code over without or just minimal changes!
1) wrappers for popular native APIs and frameworks to simplify porting existing code: apart from the 'expected' C and C++ runtime wrappers, emscripten also offers wrappers for GL, GLES 1..3, EGL, GLFW, GLU, GLUT, GLEW, SDL and OpenAL, these generally translate (and sometimes emulate) the behaviour of the native APIs to the underlying HTML5 APIs (e.g. GL to WebGL, SDL Audio and OpenAL to WebAudio, etc...)
2) there are also smaller emscripten-specific APIs that are essentially minimal shims around HTML5 APIs, for instance to initialize a WebGL context, one could either use the EGL or SDL wrappers which add some code bloat and don't directly map to WebGL initialization attributes, or one can use the emscripten_webgl_*() functions which map directly to the WebGL context initialization process, there are also similar small shims for input, XMLHttpRequest, WebWorkers and others, the advantage of these direct HTML5 shims is that they introduce less code bloat, but they don't exist anywhere outside the emscripten platform
3) emscripten also provides a number of different ways to easily write your own shims to call from C/C++ into JS, and the other way around, even with marshalling complex C++ data types (see: https://kripken.github.io/emscripten-site/docs/porting/conne...)
I really like that emscripten is much more than just a compiler toolchain with C/C++ runtime, there has been a tremendous amount of work to make it as simple as possible to port existing code over without or just minimal changes!