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Bun.js in 2025: Fast, Modern, but mind the compatibility gaps

·3 mins

🔥 What is Bun? #

Bun.js is a modern JavaScript runtime like Node.js — but faster, and with batteries included. Built in Zig, it bundles:

  • A JS/TS runtime (using JavaScriptCore, not V8)
  • A native transpiler (TS/JSX/ESM)
  • A test runner (bun test)
  • A fast package manager (bun install)
  • Built-in HTTP server (Bun.serve)
  • Native SQLite & Postgres clients

Think: Node + npm + ts-node + Jest + esbuild + nodemon… all in one binary.

🚀 Why Bun? #

  • Blazing fast 🏎️ — startup, I/O, HTTP, and test runs all faster than Node/Deno (AppSignal Blog).
  • All-in-one toolkit — runtime, bundler, test runner, bunx, and built-in DB clients.
  • TypeScript + ESM by default, hot-reload dev mode, Web APIs like fetch, Request, etc.
  • High Node.js compatibility, but not perfect — key ecosystem gaps remain.

⚠️ What Doesn’t Work Well (Yet) #

Some of the most-used Node.js frameworks are still flaky:

  • Express: Mostly works, but middleware/plugins can be buggy.
  • NestJS: Basic apps work (with decorators), but some features still fail.
  • Fastify: Historically unsupported, compatibility improving but still not reliable.

✅ Bun-Native Alternatives #

🔹 Elysia — Web Framework #

Fast, ergonomic, and built for Bun.

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import { Elysia } from 'elysia'

const app = new Elysia()

app.get('/', () => 'Hello from Bun!')
app.get('/hello/:name', ({ params }) => `Hi, ${params.name}!`)

app.listen(3000)

🔹 Drizzle ORM + bun:sqlite #

Type-safe, minimal, and Bun-native.

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import { Database } from 'bun:sqlite'
import { drizzle } from 'drizzle-orm/bun-sqlite'
import { sqliteTable, text, integer } from 'drizzle-orm/sqlite-core'

const db = drizzle(new Database('app.db'))

const users = sqliteTable('users', {
  id: integer('id').primaryKey(),
  name: text('name'),
  age: integer('age'),
})

await db.insert(users).values({ name: 'Alice', age: 30 })

const result = await db.select().from(users)
console.log(result)

🧪 Testing with Bun #

Snapshot support, mocks, filters, blazing speed.

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test("math works", () => {
  expect(2 + 2).toBe(4)
})

📌 Real Use Cases #

  1. High-performance APIs
  2. CLI tools
  3. MVPs and prototypes
  4. Fullstack apps with SQLite/Postgres

👥 Who’s Using Bun? #

  • Anthrop\ic
  • Typeform
  • Midjourney
  • And this node community thread in Reddit has multiple opinions of developers:

    “I’ve used Bun in production… idiosyncratic bugs showed up.”
    (Reddit thread)

✅ When to Choose Bun #

Use it if:

  • Starting new project
  • You want speed & simplicity
  • You are prepared to use Elysia, Drizzle, native tools

Avoid it if:

  • You depend on common frameworks Fastify/Nest/Express internals
  • Need Node’s entire ecosystem maturity

🧰 Getting Started #

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curl -fsSL https://bun.sh/install | bash
bun init
bun add elysia drizzle-orm
bun run index.ts

🧠 Final Thoughts #

Bun is fun and fast — but don’t treat it like a Node drop-in. Stick to Bun-native tools like Elysia and Drizzle, and it’ll feel amazing. Avoid legacy baggage, and you’ll get blazing DX and prod performance.