fix: avoid stray comma when required is set but properties is empty

[imalbi]

Problem

A response schema with required set but no properties map (e.g. what z.toJSONSchema produces for z.record(enum, V)) serializes with a stray leading comma:

const schema = {
  type: 'object',
  properties: {
    obj: {
      type: 'object',
      propertyNames: { type: 'string', enum: ['a', 'b'] },
      additionalProperties: { type: 'number' },
      required: ['a', 'b']
    }
  }
}
build(schema)({ obj: { a: 1, b: 2 } })
// {"obj":{,"a":1,"b":2}}

HTTP 200 from Fastify, but JSON.parse fails client-side.

Root cause

The optimization at index.js:389 skips the runtime comma check when requiredProperties.length > 0, on the assumption that the first declared property will anchor the comma logic. When properties is empty (or absent), the loop body never runs and addComma is set to unconditional immediately after, so the additionalProperties branch at index.js:484 emits a separator before its first entry.

Fix

Tighten the condition to propertiesKeys.length > 0 && requiredProperties.includes(propertiesKeys[0]). The sort just above (index.js:367-373) moves required-and-declared keys to the front, so this captures exactly when the "first declared property anchors the comma" premise actually holds. It falls through to the existing runtime addComma flag path in three cases that the old condition handled incorrectly:

1. properties is empty (record-style schema, only additionalProperties)
2. required only names keys that are not in properties
3. (Trivially) requiredProperties is empty — already handled before

No behaviour change for any schema where at least one declared property is required.

Tests

Three regression tests covering the canonical bug shapes:

• test/additionalProperties.test.js: required + additionalProperties with no properties map (the original z.record-style trigger).
• test/additionalProperties.test.js: required key not in properties, with additionalProperties: true.
• test/patternProperties.test.js: same shape with patternProperties (symmetric branch at index.js:484).

Each test fails on the respective baseline (main for #1, the first proposed fix for #2 and #3) and passes after this patch. Full suite: 476/476 green.

Benchmark

Ran npm run benchmark 3 times on main and 3 times on the fix branch. All scenario ranges overlap; absolute mean deltas ≤ 1.1%, within natural inter-run variance (±0.7% to ±2.0%). The modified branch is not exercised by any standard scenario, so no runtime perf change is expected by construction.

After tightening the guard further (to also handle required keys not in properties), re-ran 3 vs 3 between the first fix and the tightened version. Absolute mean deltas ≤ 1.7%, still within inter-run variance. By transitivity, the tightened fix is also performance-neutral vs main.

Related

• Schema serialization of anyOf/oneOf properties are parsed incorrectly #642 / fix: anyOf produces invalid json due to extra comma #651: same class of bug (extra-comma in object serialization), different branch.
• feat!: drop required property support #530: proposed dropping required support entirely. This fix is a smaller targeted patch in the same area while that broader change is on hold.

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Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Copilot]

Pull request overview

Fixes an invalid-JSON edge case in the object serializer where a schema can emit a leading comma when required is set but there are no declared properties, affecting record-style schemas using additionalProperties.

Changes:

• Tighten the “skip first comma” optimization in buildInnerObject to avoid unconditional comma insertion when properties is empty.
• Add a regression test covering required + additionalProperties with no declared properties on the nested object.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
index.js	Adjusts comma-optimization guard in object serialization to prevent leading commas in an edge case.
test/additionalProperties.test.js	Adds a regression test ensuring the affected schema shape produces valid JSON.

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