fix: treat null as JSON-serializable (isJSONSerializable(null) throws)

[DanMat]

Closes #571.

isJSONSerializable(null) currently throws Cannot read properties of null (reading 'buffer'):

• t === null is dead code — typeof always returns a string, never null.
• So null isn't caught there, falls past the t !== "object" guard, and reaches value.buffer, which throws.

null is valid JSON (JSON.stringify(null) === "null"), so it should return true. This replaces the dead t === null check with value === null, catching null before the .buffer access.

Also adds test/utils.test.ts — the util had no direct unit test — covering null, primitives, plain objects/arrays, and FormData/URLSearchParams. All existing tests still pass; lint clean.

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Disclosure: this change was written with AI assistance. I've reviewed, tested, and understand it, and I'll maintain it.

Summary by CodeRabbit

• Bug Fixes

  • Corrected JSON serialization checks so null is recognized as a supported value without causing errors.

• Tests

  • Added coverage for primitives, objects, arrays, custom serialization, form-related types, URL search parameters, and unsupported values.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 5534bc66-782e-4fc2-9a4f-abf86fa76372

📥 Commits

Reviewing files that changed from the base of the PR and between 1dbc37f and 363d92f.

📒 Files selected for processing (2)

• src/utils.ts
• test/utils.test.ts

Included review availability: Your plan includes up to 8 reviews per rolling hour; 7 remain after this review.

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📝 Walkthrough

Walkthrough

isJSONSerializable now correctly handles null without throwing. Vitest coverage validates null, supported JSON values, custom toJSON, and unsupported values.

Changes

JSON serialization validation

Layer / File(s)	Summary
Null handling and serialization coverage src/utils.ts, test/utils.test.ts	isJSONSerializable now accepts null. Tests cover JSON-serializable values and reject undefined, bigint, functions, FormData, and URLSearchParams.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to 363d9

This change makes null correctly report as JSON-serializable and adds focused coverage for the utility; no actionable merge-blocking risk remains beyond normal checks and review.

🚥 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 clearly states that the change fixes null handling in isJSONSerializable.
Linked Issues check	✅ Passed	The code fixes the null check that caused isJSONSerializable(null) to throw and adds direct test coverage for the reported bug [#571].
Out of Scope Changes check	✅ Passed	The implementation and tests are limited to the null-handling bug and its required coverage.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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