Really Knowing JSON: RFC 8259, Number Precision, Duplicate Keys, JSONC, JSON5 and JSON Lines
Because RFC 8259 leaves duplicate keys and number precision undefined, Node.js, Python, jq and .NET can read the same JSON differently: all four take the last value of a repeated key and only JavaScript rounds an ID above 2^53. We show BOMs, NaN, leading zeros, JSONC in appsettings.json, JSON5 and a JSON Lines version of Bookstore's order export with real output.
15 Nov 2026
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16 min
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