JSON: recursively transform null to undefined

[mfulton26]

Is there an easier way to do this?

type JsonWithNullAsUndefinedSchema = z.ZodUnion<[
    z.ZodString,
    z.ZodNumber,
    z.ZodBoolean,
    z.ZodPipe<z.ZodNull, z.ZodTransform<undefined, null>>,
    z.ZodArray<z.ZodLazy<JsonWithNullAsUndefinedSchema>>,
    z.ZodRecord<z.ZodString, z.ZodLazy<JsonWithNullAsUndefinedSchema>>,
]>;
const schema = z.lazy((): JsonWithNullAsUndefinedSchema =>
    z.union([
        z.string(),
        z.number(),
        z.boolean(),
        z.null().transform(() => undefined),
        z.array(schema),
        z.record(z.string(), schema),
    ])
);

Could something like reviver from JSON.parse() be added to z.json() options?

const schema = z.json({ revive: (_key, value) => value ?? undefined });

[inkeep-ai-assistant[bot]]

There is currently no built-in way in Zod (v4) to pass a custom reviver function (like the one in JSON.parse()) directly to a z.json() or z.jsonString() parser, because such an option does not exist in the official APIs(1). The approach you showed—using a recursive schema with .transform(() => undefined) for z.null()—is valid and aligns with how Zod expects you to handle these transformations(2).

There is an open feature request for a z.jsonString(schema) that would allow parsing a JSON string and validating the result, but it does not mention a revive option as part of its API(1). You still need to handle null-to-undefined conversion with transforms in your schema as you are currently doing.

📚 Sources:

1. Feature: Add z.jsonString schema primitive
2. Zod

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