fix(v4): emit falsy prefault values in toJSONSchema (#5824)

[mixelburg]

What

Fixes #5824

The toJsonSchema function used a truthiness check (result.schema._prefault) when deciding whether to emit a prefault value as a JSON Schema default. This silently dropped falsy prefault values like false, 0, and "".

How

Changed the check from:

if (ctx.io === "input" && result.schema._prefault)

to:

if (ctx.io === "input" && "_prefault" in result.schema)

This ensures all prefault values — including falsy ones — are correctly emitted as JSON Schema defaults.

Reproduction

// Before: default is omitted (wrong)
z.boolean().prefault(false).toJSONSchema({ io: "input" });
// { type: "boolean" }

// After: default is included (correct)
z.boolean().prefault(false).toJSONSchema({ io: "input" });
// { type: "boolean", default: false }

Tests

Added a test case for falsy prefaults (false, 0, ""). All 3713 existing tests pass.

[pullfrog]

Reviewed PR #5893 — approved with no issues. The fix correctly replaces a truthiness check (result.schema._prefault) with a property existence check ("_prefault" in result.schema), ensuring falsy prefault values (false, 0, "") are emitted as JSON Schema defaults. Tests pass.

Task list (4/4 completed)

• Read the PR diff and understand the changes
• Read the modified source file for full context
• Verify correctness of the fix and test coverage
• Submit review

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[pullfrog]

TL;DR — Fixes toJSONSchema silently dropping falsy prefault values (false, 0, "") by replacing a truthiness check with a property-existence check. Closes #5824.

Key changes

• Fix falsy _prefault detection in toJSONSchema — Replace result.schema._prefault (truthiness) with "_prefault" in result.schema (existence) so that false, 0, and "" are emitted as JSON Schema default values.
• Add tests for falsy prefault values — Cover z.boolean().prefault(false), z.number().prefault(0), and z.string().prefault("") with inline snapshots.

_{Summary ｜ 2 files ｜ 1 commit ｜ base: main ← fix/falsy-prefault-to-json-schema}

Before: toJSONSchema checked result.schema._prefault with a truthiness test, so falsy values like false, 0, and "" were silently skipped — the generated JSON Schema had no default field.
After: The check uses "_prefault" in result.schema, so any assigned prefault value — including falsy ones — is correctly emitted as the JSON Schema default.

The root cause is a single-character class of bug: using a value as a boolean guard when the value's domain includes falsy members. The fix is minimal — one conditional changed from truthiness to in — and the test covers all three common falsy types.

to-json-schema.ts · to-json-schema.test.ts

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[pullfrog]

Reviewed — no issues found.

Task list (4/4 completed)

• Read the PR diff and understand the changes
• Read the modified source file for full context
• Verify correctness of the fix and test coverage
• Submit review

  ^{｜ View workflow run ｜ Using Claude Opus ｜ 𝕏}

[colinhacks]

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