Silent moment for those of us using niche languages to meet production requirements in environments that do not allow third-party code and do not have JSON parsing in the std lib...
You can get an implementation working with a fairly high level of confidence that it's right.
If it's not fast enough, make a pretty printer for your AST. Then do a CPS transform (by hand) on your library and parser, so you can make the stack explicit. Make sure the transformed version pretty prints exactly the same way.
Then make a third version that prints out the code that should run when parsing a document, rather than doing the parsing directly. You'll get a big case switch for each grammar you want to parse. Your pretty printer will help you find many bugs.
It's a pretty achievable path to get your grammar correct, and then get a specialized parser for it.
A modern JS implementation should also have a JSON parser
[1] http://winterbe.com/posts/2014/04/05/java8-nashorn-tutorial/
Or the other way around. Think about something like MUMPS.