The 1.0.4 release of Grammars2JSON has a main method with a hard-coded input file name, rather than honoring command line arguments.
It is unfortunate because now it is referred to from the exificient.js demo web page, and the demo requires access to a generated grammar file.
https://exificient.github.io/javascript/
A grammar can be generated by running Grammars2JSON under jdb and setting a breakpoint at line 672, then storing the desired file name into xsd. But it is a bit of distraction from the demo.