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Bun Runtime Setup Guide: Migrating from Node.js and Practical Patterns

#bunjs #javascript #typescript #node

What is Bun: Differences from Node.js

Bun is a tool that integrates JavaScript runtime, bundler, and package manager into one, officially released in 2023. Built with Zig on the JavaScriptCore engine (Safari's JS engine), it operates 2-3x faster than Node.js.

Why Bun is fast comes down to architecture. Startup time is about 1/3 of Node.js, native HTTP server throughput is about 4x Express, and bun install is about 25x faster than npm.

Best use cases: Script execution, internal tools, microservices, TypeScript projects

Installation and Initial Setup 

curl -fsSL https://bun.sh/install | bash
bun --version  # 1.1.x
bun init
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bunfig.toml (Bun-specific configuration):

[install]
lockfile = "bun.lockb"

[test]
timeout = 10000
coverage = true
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Bun HTTP Server: High-Speed Web Server 

const server = Bun.serve({
  port: parseInt(process.env.PORT ?? "3000"),

  async fetch(req: Request): Promise<Response> {
    const url = new URL(req.url);

    if (url.pathname === "/health") {
      return Response.json({ status: "ok", uptime: process.uptime() });
    }

    if (url.pathname.startsWith("/api/")) {
      return handleApi(req, url);
    }

    return new Response("Not Found", { status: 404 });
  },

  error(error: Error): Response {
    return Response.json({ error: "Internal Server Error" }, { status: 500 });
  },
});
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Native File Operations 

// Bun's file API is simpler than Node.js
async function readJsonFile<T>(path: string): Promise<T> {
  return Bun.file(path).json<T>();
}

async function writeJsonFile(path: string, data: unknown): Promise<void> {
  await Bun.write(path, JSON.stringify(data, null, 2));
}

const content = await Bun.file("./config.txt").text();
await Bun.write("./output.txt", "Hello, Bun!");
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Bun Test Runner 

import { describe, it, expect, mock } from "bun:test";

describe("Users API", () => {
  it("GET /api/users returns user list", async () => {
    const req = new Request("http://localhost/api/users");
    const res = await handleApi(req, new URL(req.url));
    const body = await res.json();

    expect(res.status).toBe(200);
    expect(body).toHaveLength(1);
  });
});

// Snapshot testing
it("generates correct HTML", () => {
  const html = renderComponent({ title: "Test" });
  expect(html).toMatchSnapshot();
});
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Native SQLite Integration 

import { Database } from "bun:sqlite";

const db = new Database("myapp.db", { create: true });
db.run("PRAGMA journal_mode = WAL");

db.run(`
  CREATE TABLE IF NOT EXISTS users (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    name TEXT NOT NULL,
    email TEXT UNIQUE NOT NULL
  )
`);

const insertUser = db.prepare("INSERT INTO users (name, email) VALUES ($name, $email)");

// Transaction
const createUsers = db.transaction((users: Array<{name: string; email: string}>) => {
  for (const user of users) {
    insertUser.run({ $name: user.name, $email: user.email });
  }
});

createUsers([
  { name: "Alice", email: "alice@example.com" },
  { name: "Bob", email: "bob@example.com" },
]);
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Migration from Node.js 

# Delete package-lock.json and reinstall with bun
rm -f package-lock.json yarn.lock
bun install  # generates node_modules and bun.lockb
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Bun is particularly powerful for "scripts that need speed, internal tools, and TypeScript projects." Since it's not fully compatible with Node.js, migrating large existing projects needs care, but it's a solid choice for new projects.

This article is from the Claude Code Complete Guide (7 chapters) on note.com.
myouga (@myougatheaxo) - VTuber axolotl. Sharing practical AI development tips.

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