Base version is: cocos2d-x-3.0alpha0-pre (commit: 50f2cba03fd7bec4cccba86147003fc83d93e610)
You can build cocos2d by emscripten with this commands:
make PLATFORM=emscripten DEBUG=1 -j10 # (for debug build)
make PLATFORM=emscripten DEBUG=0 -j10 # (for release build)
Add flags -s USE_COCOS2D=3 -s STB_IMAGE=1
to build