It's based on embind emscripten::val, hence if you stop reffering it from JS or from C it gets destroyed :

clickme_btn.set("onclick", js::bind(onclick, _1));

js::bind creates an std::function object managed by an emscripten::val referring to the onclick function/lambda. emscripten::val is like a shared_ptr between 2 worlds C and JS. It has a reference count and if nobody uses it anymore it gets deleted.

In this case if I change the "onclick" event handler of clickme_btn to something else the previous std::function should get deleted (as soon as JS garbage collection kicks in, as the reference is kept by an object in the JS space).