Add reviver parameter support for parsing

[MrMeison]

Hi! Is it planned to add support for the reviver parameter in the near future? The README.md states that there is no support yet, but is it planned? What are the challenges involved?

[lahmatiy]

Yes, it’s planned. This is a complex feature because we parse JSON in fragments and assemble the resulting object piece by piece. The main challenge is providing the same execution context for the reviver callback as JSON.parse() does. In particular, this inside the callback should refer to the object (or array) being constructed, and the value argument should be fully parsed.

However, when parsing in fragments using JSON.parse(), the value may not be complete yet. Additionally, the object referenced by this must be the same across all callback invocations and for the final resulting value. The current implementation does not guarantee this.

In short, introducing reviver support is non-trivial and would require revisiting the current implementation. It also needs to avoid any performance degradation in cases where reviver is not used. I can’t provide an ETA, but I hope it happens one day. I’m planning to introduce support for JSONL/NDJSON in the coming months, and I may introduce reviver support alongside that.