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A comment key works for commenting JSON objects, but not for commenting specific keys on an object or values in an array.

I don’t find that a huge limitation personally. I don’t think data formats should allow comments on values, that is what docs are for.

JSON supports numbers of arbitrary and just fine. It's just many reader/writers of JSON does not, and those are so ubiquitous that it's often not practical to use.

AFAIK, json does not support NaN and Inf which can be a pain sometimes

Unfortunately in practice you can’t rely on large numbers being passed through correctly because of this, unless you control both ends and don’t use js.

It's pretty inconvenient and ends up in the data, rather than being external to it.