@indutny 
It gets handled in the same tick and propagates the exception to the caller, see [here](https://github.com/joyent/node/pull/6987/files#diff-e676e0d24e582043647fd35a459ca91fR183). A bit hacky, but the only alternative I see is to throw directly from C++, duplicating the `onerror` function in zlib.js, which doesn't seem easy.