fix(utils): handle null in isJSONSerializable

[MFA-G]

Summary

isJSONSerializable(null) currently throws:

TypeError: Cannot read properties of null (reading 'buffer')

null falls through every early branch and reaches the value.buffer check (src/utils.ts line 31), where reading a property off null throws.

There is also a dead-code branch: typeof value can never return null, so the t === null part of the primitive check on line 22 never matches.

Changes

• Add an explicit value === null early return. null is JSON serializable (JSON.stringify(null) === "null"), so it returns true.
• Remove the dead t === null condition from the primitive check.
• Add test/utils.test.ts with unit tests for isJSONSerializable covering null, undefined, primitives, plain objects, toJSON objects, arrays, and non-serializable values (functions, symbols, bigint, typed arrays, FormData, URLSearchParams).

Validation

• pnpm lint passes (eslint + prettier).
• vitest run passes (34 tests, including the 6 new ones).
• Confirmed the new null test fails against the current code (reproduces the TypeError) and passes with this fix.

Closes #571

Summary by CodeRabbit

• Bug Fixes

  • Corrected JSON serializability checks so null is now treated as serializable.
  • Improved handling of values that should not be considered JSON-safe, reducing incorrect results.

• Tests

  • Added coverage for serializable and non-serializable values.
  • Verified support for null, objects, arrays, and toJSON-based values, along with rejection of unsupported types.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 29e83460-96ad-4c6e-8b31-061b6c6dca33

📥 Commits

Reviewing files that changed from the base of the PR and between dfbe3ca and 12274d1.

📒 Files selected for processing (2)

• src/utils.ts
• test/utils.test.ts

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📝 Walkthrough

Walkthrough

isJSONSerializable now treats null as serializable before type checks. A new test suite covers null, primitives, objects, arrays, toJSON-based values, and several non-serializable inputs.

Changes

Null serialization behavior

Layer / File(s)	Summary
Null handling and serializability tests src/utils.ts, test/utils.test.ts	isJSONSerializable returns true for null before inspecting typeof, and the Vitest suite checks null, primitives, plain objects/arrays, toJSON support, and rejected non-JSON values.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 I hopped to the null and tapped the gate,
“You’re JSON-safe,” said the carrot crate.
Primitives twinkled, arrays stayed bright,
Odd values stayed out, all snug and right.
Hooray for tidy burrows tonight!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: fixing null handling in isJSONSerializable.
Linked Issues check	✅ Passed	The code and tests address #571 by making null serializable and preventing the null buffer TypeError.
Out of Scope Changes check	✅ Passed	The changes are scoped to the null-serialization fix and its test coverage, with no unrelated edits.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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