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description Use Bun instead of Node.js, npm, pnpm, or vite.
globs *.ts, *.tsx, *.html, *.css, *.js, *.jsx, package.json
alwaysApply false

Default to using Bun instead of Node.js.

  • Use bun <file> instead of node <file> or ts-node <file>
  • Use bun test instead of jest or vitest
  • Use bun build <file.html|file.ts|file.css> instead of webpack or esbuild
  • Use bun install instead of npm install or yarn install or pnpm install
  • Use bun run <script> instead of npm run <script> or yarn run <script> or pnpm run <script>
  • Bun automatically loads .env, so don't use dotenv.

APIs

  • Bun.serve() supports WebSockets, HTTPS, and routes. Don't use express.
  • bun:sqlite for SQLite. Don't use better-sqlite3.
  • Bun.redis for Redis. Don't use ioredis.
  • Bun.sql for Postgres. Don't use pg or postgres.js.
  • WebSocket is built-in. Don't use ws.
  • Prefer Bun.file over node:fs's readFile/writeFile
  • Bun.$ls instead of execa.

Testing

Use bun test to run tests.

Bun Module Mocking

  • Treat mock.module() as process-global. In Bun, module mocks can leak across test files via the shared module cache and cause unrelated failures later in the full suite.
  • Do not mock broad/shared modules like settings-manager, telemetry helpers, or other hot imports unless there is no better seam.
  • Prefer test seams, object-level stubbing, or patching a specific exported instance over module-level mocking of transitive dependencies.
  • If you must use mock.module(), install the mock inside per-test setup, restore it in teardown, and do not assume that dynamic import() gives you a fresh copy of a previously imported module.
  • If a test only passes in isolation but fails in bun test src/tests, suspect module-mock leakage first.
import { test, expect } from "bun:test";

test("hello world", () => {
  expect(1).toBe(1);
});

Frontend

Use HTML imports with Bun.serve(). Don't use vite. HTML imports fully support React, CSS, Tailwind.

Server:

import index from "./index.html"

Bun.serve({
  routes: {
    "/": index,
    "/api/users/:id": {
      GET: (req) => {
        return new Response(JSON.stringify({ id: req.params.id }));
      },
    },
  },
  // optional websocket support
  websocket: {
    open: (ws) => {
      ws.send("Hello, world!");
    },
    message: (ws, message) => {
      ws.send(message);
    },
    close: (ws) => {
      // handle close
    }
  },
  development: {
    hmr: true,
    console: true,
  }
})

HTML files can import .tsx, .jsx or .js files directly and Bun's bundler will transpile & bundle automatically. <link> tags can point to stylesheets and Bun's CSS bundler will bundle.

<html>
  <body>
    <h1>Hello, world!</h1>
    <script type="module" src="./frontend.tsx"></script>
  </body>
</html>

With the following frontend.tsx:

import React from "react";

// import .css files directly and it works
import './index.css';

import { createRoot } from "react-dom/client";

const root = createRoot(document.body);

export default function Frontend() {
  return <h1>Hello, world!</h1>;
}

root.render(<Frontend />);

Then, run index.ts

bun --hot ./index.ts

For more information, read the Bun API docs in node_modules/bun-types/docs/**.md.