fix(response): properly serialize Array subclasses like Bun SQL results

[raunak-rpm]

Summary

Fixes #1656

Problem

When returning an Array subclass (e.g., Bun SQL query results), Elysia incorrectly returned [object Object][object Object] instead of valid JSON.

Example from issue:

const app = new Elysia()
    .get("/", async () => {
        const users = await sql`SELECT * FROM users`;  // Bun SQL returns Array subclass
        return users;
    })

Expected: [{"id":1,"name":"Alice"},{"id":2,"name":"Bob"}]
Actual: [object Object][object Object]

Root Cause

The response handlers (mapResponse, mapEarlyResponse, mapCompactResponse) used response?.constructor?.name === 'Array' to detect arrays:

switch (response?.constructor?.name) {
    case 'Array':
    case 'Object':
        return new Response(JSON.stringify(response), ...)
    // ...
    default:
        return new Response(response as any)  // <- Falls here for subclasses
}

This check fails for Array subclasses because their constructor.name is the subclass name (e.g., 'SQLResults'), not 'Array'. The response then fell through to new Response(response) which implicitly calls toString(), producing [object Object] for each element.

Proof:

class SQLResults extends Array { ... }
const results = new SQLResults({id: 1}, {id: 2})

results.constructor.name  // 'SQLResults' - NOT 'Array'!
Array.isArray(results)    // true - correctly identifies it

Solution

Added Array.isArray(response) check in the default case of all response mapping functions. This correctly identifies all arrays including subclasses, ensuring proper JSON serialization.

default:
    // ... existing checks ...
    
    // Handle Array subclasses (e.g., Bun SQL results)
    if (Array.isArray(response)) {
        set.headers['content-type'] = 'application/json'
        return new Response(JSON.stringify(response), set as any)
    }

    return new Response(response as any, set as any)

Changes

• src/adapter/bun/handler.ts: Add Array.isArray check in mapResponse, mapEarlyResponse (both branches), and mapCompactResponse default cases
• src/adapter/web-standard/handler.ts: Same changes for web standard adapter
• test/response/array-subclass.test.ts: Add 10 comprehensive tests

Testing

• ✅ All 1456 tests pass (10 new tests added)
• ✅ Verified with simulated Bun SQL results
• ✅ Content-Type header correctly set to application/json
• ✅ No regressions for regular arrays and objects

Test Coverage:

• SQL result simulation (SQLResults class)
• ORM result sets (ORMResultSet class)
• Array subclass with set headers
• Empty Array subclass
• Array subclass in beforeHandle (mapEarlyResponse)
• Nested arrays and objects
• Regular arrays still work correctly
• Plain objects still work correctly

Summary by CodeRabbit

• Bug Fixes

  • Enhanced response handling for Array subclass objects (such as database and ORM results) to ensure they are consistently serialized as JSON with proper content-type headers across all response processing paths.

• Tests

  • Added comprehensive test coverage for Array subclass serialization scenarios, including header preservation, status overrides, empty collections, nested structures, and various response mapping strategies.

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[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/elysiajs/elysia@1675

commit: 5587569

[coderabbitai]

Walkthrough

This PR adds explicit handling for Array subclasses across response-mapping functions in both Bun and web-standard adapters. When responses are Arrays, the code now returns JSON-stringified data with proper content-type headers, addressing a bug where Bun SQL results and similar Array subclasses were not being correctly serialized.

Changes

Cohort / File(s)	Summary
Response Handlers src/adapter/bun/handler.ts, src/adapter/web-standard/handler.ts	Adds Array.isArray() checks in mapResponse, mapEarlyResponse (multiple branches), and mapCompactResponse to explicitly serialize Array subclasses as JSON with application/json content-type, preventing fallthrough to generic response handling
Array Subclass Tests test/response/array-subclass.test.ts	New comprehensive test suite covering JSON serialization of Array subclasses (SQLResults, ORMResultSet), header/status handling, empty arrays, nested structures, and integration with mapEarlyResponse scenarios

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• Fix thenable fallback in mapCompactResponse causing runtime crash with undefined variable #1662: Modifies the same response-mapping functions (mapCompactResponse, etc.) in handler files, suggesting concurrent or related work on response handling logic.

Poem

🐰 Array subclasses hop with glee,
Now JSON-stringified, clear as can be!
Bun's SQL results dance free,
With proper headers for all to see! 🎉
No more [object Object] spree!

🚥 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 accurately and concisely describes the main fix: proper serialization of Array subclasses like Bun SQL results.
Linked Issues check	✅ Passed	The changes directly address issue #1656 by adding Array.isArray() checks to detect and JSON-serialize Array subclasses with correct Content-Type headers.
Out of Scope Changes check	✅ Passed	All changes are scoped to fixing Array subclass serialization in response handlers and adding comprehensive tests; no unrelated modifications detected.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

• 📝 Generate docstrings

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📜 Recent review details

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between f027642 and 5587569.

📒 Files selected for processing (3)

• src/adapter/bun/handler.ts
• src/adapter/web-standard/handler.ts
• test/response/array-subclass.test.ts

🔇 Additional comments (11)

src/adapter/bun/handler.ts (4)

143-148: LGTM! Correct fix for Array subclass detection.

The Array.isArray() check properly detects Array subclasses that constructor.name === 'Array' misses. Placement in the default branch after other type checks is appropriate.

---

288-293: LGTM!

Correctly handles Array subclasses in the mapEarlyResponse branch where headers/status/cookie are set.

---

412-418: LGTM!

Correctly handles Array subclasses in the mapEarlyResponse else branch, creating a new headers object since set isn't being used in this path.

---

537-543: LGTM!

Correctly handles Array subclasses in mapCompactResponse, following the same pattern as the existing charCodeAt JSON detection block above.

test/response/array-subclass.test.ts (3)

17-25: LGTM! Well-designed test fixture.

The SQLResults class correctly simulates Bun SQL's Array subclass behavior using Object.setPrototypeOf to ensure proper prototype chain. This is the recommended pattern for extending built-in types like Array.

---

28-38: LGTM!

Good representation of ORM-style result sets. The query and duration metadata properties correctly won't appear in the JSON output since JSON.stringify on arrays only serializes indexed elements, matching real-world ORM behavior.

---

40-213: Comprehensive test coverage for the fix.

The tests effectively cover:

• All three response mapping functions (mapResponse, mapEarlyResponse, mapCompactResponse)
• Edge cases (empty arrays, nested structures)
• Header propagation and status code handling
• Regression tests ensuring plain arrays/objects still work

src/adapter/web-standard/handler.ts (4)

175-180: LGTM!

Correctly mirrors the fix in bun/handler.ts for the web-standard adapter's mapResponse function.

---

321-326: LGTM!

Correctly handles Array subclasses in mapEarlyResponse with set headers/status/cookie.

---

446-452: LGTM!

Correctly handles Array subclasses in the mapEarlyResponse else branch with standalone headers object.

---

575-581: LGTM!

Correctly handles Array subclasses in mapCompactResponse, completing the fix across all response mapping paths.

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