fix: support ArrayBuffer serialization

[xianjianlf2]

Problem

A plain ArrayBuffer currently round-trips to an empty object, silently losing all bytes and the type:

const buf = new Uint8Array([1, 2, 3, 4]).buffer;
SuperJSON.parse(SuperJSON.stringify({ buf }));
// => { buf: {} }

SuperJSON already supports every TypedArray (including the recently added BigInt64Array/BigUint64Array, #353), but a bare ArrayBuffer isn't a plain object, array, Map/Set, Error, or registered class, so it falls through untransformed and JSON.stringify reduces it to {} — with no error or warning.

Changes

• Added an ArrayBuffer leaf transformer in src/transformer.ts (with an isArrayBuffer guard in src/is.ts), following the same convention as the existing typed-array support: the buffer is serialized as an array of byte values (JSON can't represent raw bytes) and reconstructed via new Uint8Array(bytes).buffer.
• Added a stringify & parse test case covering a non-empty and an empty ArrayBuffer, asserting the serialized form, the annotation, and byte-exact reconstruction (+24 lines in src/index.test.ts).
• Added ArrayBuffer to the README support table.

npm test (vitest): 7 files, 87 passed, 1 skipped (pre-existing), 1 todo.

Greptile Summary

This PR adds native ArrayBuffer serialization support to SuperJSON by following the existing typed-array convention: the buffer is serialized as a plain JSON array of byte values and reconstructed via new Uint8Array(bytes).buffer. Before this change, a bare ArrayBuffer would silently serialize to {}, losing all data.

• src/is.ts / src/transformer.ts: Adds isArrayBuffer guard and registers a new simpleTransformation entry that converts ArrayBuffer ↔ number[]. The rule is correctly placed after all other simple rules and does not conflict with the existing isTypedArray composite rule (since ArrayBuffer.isView(anArrayBuffer) is false).
• src/index.test.ts: Covers non-empty and empty ArrayBuffer round-trips, verifying serialized form, annotation key, instance type, and byte-exact reconstruction.

Confidence Score: 4/5

Safe to merge — the change is self-contained and the round-trip is byte-exact.

The implementation correctly follows the existing simple-transformation pattern, does not conflict with the TypedArray composite rule (since ArrayBuffer.isView returns false for a bare ArrayBuffer), and the test covers both empty and non-empty buffers with byte-exact assertions. One minor style inconsistency was noted but does not affect correctness.

No files require special attention.

Important Files Changed

Filename	Overview
src/transformer.ts	Adds ArrayBuffer to LeafTypeAnnotation union and appends a simpleTransformation rule; logic is correct and consistent with surrounding patterns
src/is.ts	Adds isArrayBuffer guard using instanceof ArrayBuffer, idiomatic and consistent with all other type guards in the file
src/index.test.ts	Adds a stringify/parse test case covering non-empty and empty ArrayBuffers with byte-exact assertions; follows the existing dontExpectEquality + customExpectations pattern correctly
README.md	Adds ArrayBuffer row to the supported-types table; accurate and complete

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant C as Caller
    participant SJ as SuperJSON
    participant T as transformer.ts
    participant IS as is.ts

    Note over C,IS: Serialization (stringify)
    C->>SJ: "stringify({ buf: ArrayBuffer })"
    SJ->>T: transformValue(ArrayBuffer)
    T->>IS: isArrayBuffer(value)
    IS-->>T: true
    T->>T: Array.from(new Uint8Array(buf))
    T-->>SJ: "{ value: [1,2,3,4], type: ArrayBuffer }"
    SJ-->>C: json string with annotation

    Note over C,IS: Deserialization (parse)
    C->>SJ: parse(jsonString)
    SJ->>T: untransformValue([1,2,3,4], ArrayBuffer)
    T->>T: new Uint8Array([1,2,3,4]).buffer
    T-->>SJ: ArrayBuffer(4)
    SJ-->>C: "{ buf: ArrayBuffer }"

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sequenceDiagram
    participant C as Caller
    participant SJ as SuperJSON
    participant T as transformer.ts
    participant IS as is.ts

    Note over C,IS: Serialization (stringify)
    C->>SJ: "stringify({ buf: ArrayBuffer })"
    SJ->>T: transformValue(ArrayBuffer)
    T->>IS: isArrayBuffer(value)
    IS-->>T: true
    T->>T: Array.from(new Uint8Array(buf))
    T-->>SJ: "{ value: [1,2,3,4], type: ArrayBuffer }"
    SJ-->>C: json string with annotation

    Note over C,IS: Deserialization (parse)
    C->>SJ: parse(jsonString)
    SJ->>T: untransformValue([1,2,3,4], ArrayBuffer)
    T->>T: new Uint8Array([1,2,3,4]).buffer
    T-->>SJ: ArrayBuffer(4)
    SJ-->>C: "{ buf: ArrayBuffer }"

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Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
src/transformer.ts:186
Minor style inconsistency: the existing `typedArrayRule` spreads its view with `[...v]` (line 256), while the new rule uses `Array.from(new Uint8Array(v))`. Both produce identical results, but using the spread form matches the established pattern in this file.

```suggestion
    v => [...new Uint8Array(v)],
```

_{Reviews (1): Last reviewed commit: "fix: preserve ArrayBuffer instead of sil..." | Re-trigger Greptile}

Greptile also left 1 inline comment on this PR.

[greptile-apps]

Minor style inconsistency: the existing typedArrayRule spreads its view with [...v] (line 256), while the new rule uses Array.from(new Uint8Array(v)). Both produce identical results, but using the spread form matches the established pattern in this file.

Suggested change

	v => Array.from(new Uint8Array(v)),
	v => [...new Uint8Array(v)],

Prompt To Fix With AI

This is a comment left during a code review.
Path: src/transformer.ts
Line: 186

Comment:
Minor style inconsistency: the existing `typedArrayRule` spreads its view with `[...v]` (line 256), while the new rule uses `Array.from(new Uint8Array(v))`. Both produce identical results, but using the spread form matches the established pattern in this file.

```suggestion
    v => [...new Uint8Array(v)],
```

How can I resolve this? If you propose a fix, please make it concise.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!