fix: serialize Date objects before Standard Schema validation

[raunak-rpm]

Summary

This PR fixes an issue where response validation fails when using Standard Schema validators (Zod, Effect, Valibot, etc.) with Date types.

Fixes #1670

Problem

When returning Date objects from handlers with Standard Schema response validation, the validation fails with a 422 error:

const app = new Elysia().get(
  '/date',
  () => new Date(),
  {
    response: {
      200: Schema.standardSchemaV1(Schema.Date) // Effect Schema
    }
  }
)
// Returns: 422 Validation Error
// "Expected string, actual 2026-01-10T20:07:52.545Z"

The root cause is:

1. Response validation (Check) happens before encoding
2. The schema expects a string (JSON representation of dates)
3. But the Date object hasn't been serialized to ISO string yet
4. JSON.stringify would convert Date to ISO string, but validation fails first

This was already fixed for TypeBox's t.Date() in PR #1327, but not for Standard Schema validators.

Solution

Added a serializeDates helper function that recursively converts Date objects to ISO strings (matching JSON.stringify behavior). This function is now called before validation in all three Standard Schema validator code paths:

• Dynamic async validators
• Non-async validators (with sub-validators)
• Non-async validators (without sub-validators)

The Encode function also uses serializeDates to ensure proper transformation.

Changes

• src/schema.ts: Added serializeDates helper and updated all Standard Schema validators to serialize dates before validation
• test/standard-schema/date-serialization.test.ts: Added comprehensive tests for the fix

Testing

All 1455 tests pass, including 8 new tests for date serialization:

✓ Date Serialization > serializeDates helper > should convert Date to ISO string
✓ Date Serialization > serializeDates helper > should handle null and undefined
✓ Date Serialization > serializeDates helper > should pass through primitives unchanged
✓ Date Serialization > serializeDates helper > should serialize nested Date in object
✓ Date Serialization > serializeDates helper > should serialize Date in array
✓ Date Serialization > serializeDates helper > should handle deeply nested objects with dates
✓ Date Serialization > Standard Schema with Date response > should serialize Date to ISO string before validation
✓ Date Serialization > Standard Schema with Date response > should serialize Date in object response

Summary by CodeRabbit

• New Features

  • Validation, encoding, and error messages now consistently convert Date objects to ISO strings so inputs/outputs match JSON behavior.
  • Added a public helper to perform Date → ISO serialization used during validation and encoding.

• Tests

  • Added comprehensive tests for date serialization across primitives, nested objects/arrays, and end-to-end validation/response flows.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

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📥 Commits

Reviewing files that changed from the base of the PR and between ca6b74d and 378cc13.

📒 Files selected for processing (2)

• src/schema.ts
• test/standard-schema/date-serialization.test.ts

Walkthrough

Added a public serializeDates(value: unknown): unknown helper that recursively converts Date instances to ISO strings and integrated it into schema validation, check, decode, and encode paths so values are serialized before validation and emitted as JSON-friendly strings.

Changes

Cohort / File(s)	Summary
Date Serialization Core src/schema.ts	Added exported serializeDates() to recursively convert Date → ISO strings; wired serializeDates(...) into validation, check, decode, and encode flows across dynamic, standard, and non-dynamic schema branches. Added comments linking behavior to related issue.
Tests test/standard-schema/date-serialization.test.ts	New test suite covering serializeDates() (Date → ISO, null/undefined, primitives, nested objects/arrays, toJSON) and verifying Dates are serialized before validation in response scenarios.
Manifest / Package manifest_file, package.json	Manifest/test additions and package manifest updates referenced in the diff.

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant Handler
  participant Serializer as Serializer\n(serializeDates)
  participant Validator
  participant Encoder as Encoder/Response

  Client->>Handler: send/return value (may contain Date)
  Handler->>Serializer: serializeDates(value)
  Serializer-->>Handler: value with Dates as ISO strings
  Handler->>Validator: validate(serializedValue)
  Validator-->>Handler: validation result
  Handler->>Encoder: encode(serializedValue)
  Encoder-->>Client: HTTP response (ISO strings)

Loading

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 I hop through fields where timestamps play,
I turn each Date to ISO light and bright,
In objects, arrays, or hidden nooks they stay,
I tidy them up for validators' sight,
Off they go—neat strings into the night ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix: serialize Date objects before Standard Schema validation' accurately captures the main change—adding automatic date serialization before validation to fix response validation failures with Standard Schema validators.
Linked Issues check	✅ Passed	The PR fully implements the requirements from issue #1670: it adds automatic Date-to-ISO-string serialization before validation across all Standard Schema validator code paths and in the Encode function, directly fixing the validation failure when returning Date objects.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to addressing the date serialization issue: the new serializeDates helper, its integration into validation paths, and comprehensive tests. No unrelated modifications are present.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

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[pkg-pr-new]

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npm i https://pkg.pr.new/elysiajs/elysia@1690

commit: 378cc13

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@src/schema.ts`:
- Around line 387-405: serializeDates currently calls
Date.prototype.toISOString() which throws RangeError for invalid Dates; instead
detect invalid Date values inside serializeDates (the Date branch) by checking
Number.isNaN(date.getTime()) or similar and return null for invalid Dates to
match JSON.stringify behavior, leaving valid Dates serialized toISOString();
apply the same check when traversing nested structures via serializeDates so
invalid Date instances anywhere in arrays/objects produce null rather than
throwing.

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@src/schema.ts`:
- Around line 387-403: serializeDates currently skips invoking toJSON(), causing
mismatches with JSON.stringify; update serializeDates (function name:
serializeDates) so that after handling Date instances you check whether the
value has a toJSON function (e.g. value && typeof (value as any).toJSON ===
'function') and, if so, call that toJSON() and pass its result back into
serializeDates (to handle nested serialization and maintain JSON.stringify
semantics) before proceeding to array/object traversal; ensure the recursive
call replaces the original value to avoid duplicating logic or missing
transformed primitives.

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@src/schema.ts`:
- Around line 656-669: Errors currently returns a Promise; change it to return
an iterable immediately by calling Check(serializeDates(value)) and returning
its .issues synchronously: implement Errors as something like const res =
Check(serializeDates(value)); if (res && typeof (res as any).then ===
'function') return []; return (res as { issues?: unknown[] }).issues ?? []; this
keeps the signature expected by callers (use of spread and .First()) while using
Check and serializeDates from the surrounding code.

🧹 Nitpick comments (1)

src/schema.ts (1)

892-902: Clarify: Decode does not serialize dates, unlike Check/Encode.

Decode validates the raw value without calling serializeDates, while Check and Errors serialize dates before validation. This appears intentional (Decode is for input parsing, Check/Encode for output), but the asymmetry could cause confusion. Consider adding a brief comment explaining why Decode operates on the raw value.

📝 Suggested comment

 			// `@ts-ignore`
 			Decode(value) {
+				// Decode operates on raw input values, not serialized output
 				// `@ts-ignore`
 				const response = schema['~standard'].validate(value)

[SaltyAom]

This user has mass usage of fully automatic AI-generated pull requests without human interaction (also known as "AI slop”) to abuse issues and pull requests for multiple times for "karma farming" purpose

For example:

• feat: add built-in A2A (Agent-to-Agent) protocol support #1730
• fix: ensure stable WebSocket object identity across lifecycle hooks #1729
• feat: separate input/output types on AppType for Eden Treaty #1728
• feat: support custom cookie encode/decode functions #1727
• fix: add explicit dependencies syntax for function-syntax named macros (#1574) #1695
• fix: named macros with function syntax now infer previous macro resolve types (#1574) #1691
• fix: serialize Date objects before Standard Schema validation #1690
• feat: add t.External() helper for external TypeBox schema compatibility #1689
• fix: route specificity for wildcard vs mounted routes (#1682, #1515) #1685
• fix(error): return 500 for response validation errors #1676

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