Returning non-JSON serializable response when using Elysia with Bun SQL

[JagritGumber]

What version of Elysia is running?

1.4.21

What platform is your computer?

Microsoft Windows NT 10.0.26100.0 x64

What environment are you using

Bun: 1.3.0

Are you using dynamic mode?

No

What steps can reproduce the bug?

Use this script

import { Elysia } from "elysia";
import { SQL } from "bun";

// Connect to a local postgres instance
// Make sure to set these env vars or change the values to match your DB
const sql = new SQL({
    url: process.env.DATABASE_URL || "postgres://postgres:postgres@localhost:5432/postgres",
});

// Setup a simple table and data for demonstration
try {
    await sql`CREATE TABLE IF NOT EXISTS elysia_repro_users (id SERIAL PRIMARY KEY, name TEXT)`;
    // Insert some data if empty
    const count = await sql`SELECT COUNT(*) as count FROM elysia_repro_users`;
    if (count[0].count === 0n) { // Bun SQL returns BigInt for count
         await sql`INSERT INTO elysia_repro_users (name) VALUES ('Alice'), ('Bob')`;
    }
} catch (e) {
    console.error("Failed to setup DB (ignore if just checking reproduction code structure):", e);
}

const app = new Elysia()
    .get("/", async () => {
        // Fetch data using Bun's native SQL
        // This returns an array of objects (Postgres rows)
        const users = await sql`SELECT * FROM elysia_repro_users`;
        
        console.log("Users from DB:", users);
        console.log("Type of users:", typeof users);
        console.log("Is array?", Array.isArray(users));
        
        // The Issue:
        // Elysia should serialize this as JSON.
        // However, if the response is coming as [object Object][object Object],
        // it implies Elysia is using .toString() instead of JSON.stringify()
        // or the Content-Type is inferred incorrectly.
        return users;
    })
    .listen(3000);

console.log(`🦊 Elysia is running at ${app.server?.hostname}:${app.server?.port}`);
console.log(`Querying database: ${process.env.DATABASE_URL || "postgres://postgres:postgres@localhost:5432/postgres"}`);

What is the expected behavior?

Response should be valid JSON:

[{"id":1,"name":"Alice"},{"id":2,"name":"Bob"}]

What do you see instead?

Response is a string concatenation of object representations:

[object Object][object Object]

Additional information

Workaround

Manually serializing the response or using a mapResponse hook fixes the issue.

Example workaround in Elysia app:

new Elysia()
  .mapResponse(({ responseValue, set }) => {
    if (typeof responseValue === "object" && responseValue !== null) {
       // Force JSON serialization
       const text = JSON.stringify(responseValue);
       set.headers["Content-Type"] = "application/json; charset=utf-8";
       return new Response(text);
    }
  })

Have you try removing the node_modules and bun.lockb and try again yet?

No