Bun — All-in-One JavaScript Runtime

What it is

Bun is a JavaScript runtime built from scratch in Zig with JavaScriptCore (Safari's engine) instead of V8. It bundles four tools into one binary: a runtime (replaces Node.js), a bundler (replaces Webpack/esbuild), a test runner (replaces Jest), and a package manager (replaces npm/yarn/pnpm). It supports TypeScript and JSX natively without a compilation step.

Bun targets JavaScript and TypeScript developers who want faster tooling. Its startup time, install speed, and bundling performance are significantly faster than the Node.js ecosystem equivalents.

How it saves time or tokens

Bun's package manager installs dependencies up to 25x faster than npm by using hardlinks and a global cache. The runtime starts in milliseconds. The built-in test runner eliminates Jest configuration. By combining four tools into one, Bun reduces the dependency count and configuration files in your project.

How to use

1. Install Bun via curl or your package manager.
2. Use it as a drop-in replacement for Node.js, npm, and Jest.
3. Run TypeScript files directly without compilation.

# Install Bun
curl -fsSL https://bun.sh/install | bash

# Run a TypeScript file directly
bun run server.ts

# Install packages (faster than npm)
bun install

# Run tests
bun test

# Bundle for production
bun build ./src/index.ts --outdir ./dist

Example

// server.ts - HTTP server with Bun
const server = Bun.serve({
  port: 3000,
  fetch(req) {
    const url = new URL(req.url);
    if (url.pathname === '/api/hello') {
      return Response.json({ message: 'Hello from Bun' });
    }
    return new Response('Not Found', { status: 404 });
  },
});

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

Related on TokRepo

• AI coding tools — Tools that support Bun and TypeScript development
• Featured workflows — Popular developer tool configurations

Common pitfalls

• Not all Node.js APIs are implemented in Bun yet. Some npm packages that rely on Node-specific APIs (like child_process edge cases) may not work. Check Bun's compatibility page before migrating.
• Bun's test runner uses a different assertion API than Jest. Existing Jest test suites may need minor adjustments for expect() matchers.
• Global installs with bun install -g place binaries in a different path than npm. Make sure Bun's bin directory is in your PATH.