An open API implies a certain intent of the author. If there is an open API to utilize some feature, that implies an expectation that others will use it and thus an expectation of a consistent interface version to version. If you're forced to go around monkey-patching the core framework, you can expect a moving target as there is no expectation of external users thus no motivation to maintain a consistent interface between versions.

So in this case, the core framework being exposed to javascript is evidence of the expectation of external users, thus it will likely not be a moving target. If you're modifying some software, you will greatly prefer programming to an API than not.