I don't think I would be comfortable with production code that swallowed access errors by providing a guard post value. I feel like that would too easily slip through in places it shouldn't. To ensure it didn't happen you would need to include explicit checks everywhere, which sort of defeats the stated purpose of the library.

However, it is a handy feature for writing tests. I wrote a similar library for testing called magical-mock based on the python library. https://www.npmjs.com/package/magical-mock