Convert Numbers.app spreadsheet to JSON document?
I don't believe that you can directly convert a Numbers document to JSON. However, you can export a Numbers document to any number of formats, shown below.



There are a bunch of converters for CSV->JSON. I found two online converters from a quick Google search: Convert CSV and Mr. Data Converter.

This is not an ideal solution if you have a lot of these documents to convert, but I am sure that it will be easier to find effective CSV->JSON converters than Numbers->JSON converters.