Bun

What it is

Bun is an all-in-one toolkit for JavaScript and TypeScript apps, made by the oven-sh team. It ships as a single executable called bun. Its core is a fast runtime that the README calls a drop-in replacement for Node.js, built on JavaScriptCore. The same command also runs tests and package.json scripts, installs packages and bundles code. It runs TypeScript and JSX with no setup.

1. bun install reads package.json and writes bun.lock
2. bun run index.ts runs TypeScript or JSX as it is
3. bun test runs your tests in the Bun runtime
4. bun build bundles code for the browser, Bun or Node
5. bunx installs an npm package to Bun’s cache and runs it

Who it suits

JavaScript and TypeScript developers who want one tool where they now use a runtime, a package manager, a test runner and a bundler. The README says Bun’s built-in tools work in existing Node.js projects with little to no changes, so you can try one piece at a time. The install command, for example, works without Bun’s runtime.

Check the compatibility page first if your project leans on a Node.js module that it rates as partly implemented. node:https is one, and the page lists what is missing.

Where to use it

Bun runs on Linux, macOS and Windows, and the docs cover several places to put it.

• Your own machine: Linux and Windows on x64 or arm64, and macOS on Intel or Apple Silicon, through the install script, npm, Homebrew or Scoop
• Docker: the oven/bun image, with debian, slim, distroless and alpine variants
• GitHub Actions: the oven-sh/setup-bun action
• Hosting: guides for Vercel, Railway, Render, AWS Lambda, DigitalOcean and Google Cloud Run
• Coding agents: bun init writes a CLAUDE.md when it detects the Claude CLI and a Cursor rule when it detects Cursor. bun test has a quiet mode for Claude Code, Replit and any agent that sets AGENT=1

The docs also have guides for frameworks such as Next.js, Astro, Remix and SvelteKit, and for libraries such as Express, Hono and Drizzle.

How to set it up

Get it saves a zip of Mindket’s setup notes in your downloads folder, with the steps for each platform from Bun’s docs. The main route is one install line.

1. Run the install line for your system.
2. Open a new terminal window and run bun --version to check it.
3. If it says command not found, add the ~/.bun/bin folder to your PATH. The docs give the lines for macOS, Linux and Windows.
4. Start a project with bun init, or run bun install in one you already have.

curl -fsSL https://bun.com/install | bash

On Windows, run this in PowerShell instead.

powershell -c "irm bun.sh/install.ps1|iex"

npm, Homebrew and Scoop work too. The first line pipes a script into bash.

npm install -g bun
brew install oven-sh/bun/bun
scoop install bun

How to use it

Type bun, then what you want to run. Put Bun’s own flags, such as --watch, right after bun.

bun run index.tsx
bun --watch run index.tsx
bun test
bun build ./index.tsx --outdir ./build
bunx cowsay "Hello world!"

The migration guide lists the swaps from npm.

Example

You start a new project from the Quickstart. You type:

bun init my-app
cd my-app
bun run index.ts

Bun asks you to pick a template, and you choose Blank. The last command prints Hello via Bun!. Then you replace index.ts with a small server.

const server = Bun.serve({
  port: 3000,
  routes: {
    "/": () => new Response('Bun!'),
  }
});

console.log(`Listening on ${server.url}`);

You run bun run index.ts again. It prints Listening on http://localhost:3000/, and the page at that address says Bun!. The Quickstart goes on to add a package with bun add figlet, then an HTML file, then a start script.

For newcomers and experts

If you are new. A runtime is the program that runs your JavaScript outside a browser. Node.js is another one. A package manager downloads the libraries your project uses. A bundler joins many files into a few that a browser can load. Bun does all three, and it runs your tests too, from one command. Start with bun init in an empty folder and follow the Quickstart. Bun runs TypeScript and JSX with no configuration. bun init still writes a tsconfig.json for bun check and editor autocomplete.

For experts. bun install does not run the lifecycle scripts of installed dependencies unless you list the package in trustedDependencies. It also optimizes the postinstall scripts of popular packages such as esbuild and sharp by working out which scripts need to run. The --frozen-lockfile flag installs exactly what bun.lock says and exits with an error if package.json disagrees. The --production flag implies it, and bun install --frozen-lockfile --dry-run checks the lockfile without installing.

Bun’s own flags go right after bun. bun --watch run dev works, while bun run dev --watch hands the flag to your script. Watch mode restarts the process, and --hot reloads the code without restarting it. A package whose script starts with a node shebang runs on node by default. Pass --bun to run it on Bun’s runtime. bun build checks types only when you pass --check.

Good to know

Bun runs on your machine. The docs mention network use for installs, updates and canary crash reports.

• License: MIT. Bun statically links JavaScriptCore and WebKit, which are LGPL-2, and the license page lists the other libraries it links with their licenses. Read it before you ship a binary built with Bun
• Install script: the Linux and macOS line pipes a downloaded script into bash, so read it first if you prefer
• Updates: bun upgrade updates Bun. With Homebrew or Scoop, use brew upgrade bun or scoop update bun. Canary builds come from every commit to main, the docs call them untested, and they upload crash reports automatically
• Network: bun install and bunx read from the npm registry, and bunx asks it for newer versions once a day. The runtime also auto-installs packages when a project has no node_modules folder (--install defaults to auto), and --no-install turns that off. The pages Mindket read describe no other telemetry
• Dependency scripts: they stay off until you trust a package in package.json, the docs say other npm clients run them by default
• Compatibility: the docs call a package that works in Node.js but not in Bun a bug in Bun. They rate each Node.js module and global as fully implemented, partly implemented or not implemented (node:sea is not implemented)
• Removal: delete the .bun folder in your home folder, run powershell -c ~\.bun\uninstall.ps1 on Windows, or use your package manager’s uninstall