Migrating from Express

Most apps migrate by changing one import. This page lists what to check.

1. Swap the adapter

import { NestExpressApplication } from '@nestjs/platform-express';
import { BunAdapter, type NestBunApplication } from '@nestbun/platform';

const app = await NestFactory.create<NestExpressApplication>(AppModule);
const app = await NestFactory.create<NestBunApplication>(AppModule, new BunAdapter());

bun add @nestbun/platform

2. Replace the run scripts

package.json

{
  "scripts": {
    "dev": "bun --watch src/main.ts",
    "start": "bun src/main.ts",
    "test": "bun test"
  }
}

nest build and dist/ are no longer needed. Keep tsc --noEmit as a type-check step if you like.

3. Check your @Res() handlers

res is a BunResponse. It has the Express surface you already use: status(), set(), json(), send(), redirect(), cookie(), sendFile(), write(), end(), plus the Node ServerResponse methods (setHeader, writeHead, flushHeaders).

Type it explicitly if you were importing Response from Express:

import type { BunResponse } from '@nestbun/platform';

@Get('download')
download(@Res() res: BunResponse) {
  res.set('Content-Disposition', 'attachment; filename="report.csv"');
  res.send(csv);
}

4. Check third-party Express middleware

Middleware that only reads req.headers, req.body, req.query, req.method and writes through res.setHeader, res.status, res.end or res.json works as-is. That covers cors, most loggers, and most auth checks.

Middleware that mounts Express sub-routers, rewrites req.url, or reaches into IncomingMessage internals does not. Common cases and their replacements:

Express middleware	On the Bun adapter
express.static	app.useStaticAssets(dir, options)
express.json() / urlencoded()	registered by default; app.useBodyParser('text') for more
cookie-parser	read req.headers.cookie, or use Bun’s Bun.CookieMap
multer	multipart is on the roadmap; use req.native.formData() meanwhile
helmet	Nest 12’s app.useSecurityHeaders()

5. Tests

supertest needs a Node http.Server. Use the in-process adapter.fetch() instead, see Testing without a port.

6. Things that behave slightly differently

See Differences from Express. The short list: simple query-string parsing, Buffer return values sent as bytes, and route order that is first-match like Express.