fix: treat null as JSON-serializable in isJSONSerializable

[n0liu]

Since typeof null is "object", the t === null branch in isJSONSerializable was dead code (t is always a string). As a result isJSONSerializable(null) fell through to the value.buffer access and threw TypeError: Cannot read properties of null (reading 'buffer').

null is JSON-serializable (JSON.stringify(null) === "null"), so this checks value === null up front and returns true, which also removes the dead check.

No behavior change for $fetch itself — a null body is already short-circuited by the context.options.body && guard before this util is reached; the fix matters for direct/library use of the exported helper.

Resolves #571

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of null values so they are now correctly recognized as JSON-serializable.
  • Expanded validation to better distinguish serializable values from unsupported types like undefined, functions, and typed arrays.

• Tests

  • Added coverage for JSON-serializability checks across common primitives, objects, arrays, and edge cases.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 6a7fe995-9137-44c9-bc98-990ee1f77b6d

📥 Commits

Reviewing files that changed from the base of the PR and between 9102908 and 2dd0215.

📒 Files selected for processing (2)

• src/utils.ts
• test/index.test.ts

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📝 Walkthrough

Walkthrough

Fixed a bug in isJSONSerializable where the null check compared typeof value to null (always false, dead code) instead of checking value === null directly, which previously caused the function to throw or misclassify null. Added corresponding tests.

Changes

isJSONSerializable null fix

Layer / File(s)	Summary
Null check fix and tests src/utils.ts, test/index.test.ts	Changed the initial type check from comparing typeof value to null (dead code) to value === null, and added a test suite verifying null is classified as serializable without throwing, alongside classification checks for other value types.

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

Related issues: Fixes #571 – isJSONSerializable bug where null values caused a TypeError and were incorrectly classified due to a dead-code comparison.

Suggested labels: bug, tests

Suggested reviewers: pi0

🚥 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 and concisely summarizes the main fix in the PR.
Linked Issues check	✅ Passed	The code and tests directly address #571 by handling null as JSON-serializable without throwing.
Out of Scope Changes check	✅ Passed	The only changes are the null fix and targeted tests, both in scope for the reported bug.
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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