jsonwidget is a general-purpose JSON validation and manipulation library. It provides an ad hoc, just barely good enough user interface for manipulating a complicated JSON structure, and provides scaffolding to build that interface.
There are two implementations so far:
Application designers using one of these libraries have the option of providing a schema, limiting the input to a subset of valid JSON compatible with whatever application is actually consuming the JSON, or using a provided permissive schema that allows any valid JSON. The libraries are capable of dynamically generating a form is using nothing more than a schema and a JSON file as input.
The original author of JsonData (Rob Lanphier) ended substantial work on JsonData in 2012, but the extension was imported into the central MediaWiki source repository in 2013, and the JsonSchema component was imported into MediaWiki core.
As of this writing in December 2019, this extension hasn't been tested by the original author since 2012.