Json has taken over the world. Today, when any two applications communicate with each other across the internet, odds are they do so using JSON. Of the ten most popular web APIs, a list consisting mostly of APIs offered by major companies like Google, Facebook, and Twitter, only one API exposes data in XML rather than JSON. - 0b10 - Two-Bit History / The rise and rise of JSON

The ugly truth is: it was valid JavaScript. JavaScript WebApps could parse JSON with a call to eval(). No deserialization madness like XML, no need to import a parser. Just fetch a file, pass it to eval(), and you’re done. - HN

see also

• JSONC / JSON5 - flavor of Json with comments
• Confetti - multipart json
• How (not) to sign a JSON object (2019

Tools

• Bad JSON Parsers - HN
• Parsing JSON is a Minefield - I did not find two libraries that exhibit the very same behaviour. Moreover, I found that edge cases and maliciously crafted payloads can cause bugs, crashes and denial of services, mainly because JSON libraries rely on specifications that have evolved over time and that left many details loosely specified or not specified at all.
• Gron - Make JSON Greppable

Also

• {JSON} Placeholder - Free fake API for testing and prototyping.