Bun: The Fastest All-in-One JavaScript Runtime for Modern API Development

Among the latest breakthroughs in JavaScript tooling, Bun stands out for its exceptional speed and unified developer experience. Bun isn’t just a new runtime—it’s an all-in-one JavaScript toolkit that redefines performance, productivity, and simplicity for backend and API developers.

Whether you’re building high-performance APIs, web servers, or TypeScript projects, understanding Bun’s ecosystem can significantly improve your development workflow. In this guide, we’ll break down Bun’s core concepts, showcase practical examples, and explore how tools like Apidog can complement your modern API stack.

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What Is Bun? A High-Performance JavaScript Runtime

At its core, Bun is a next-generation JavaScript runtime, built to be fast, modern, and developer-centric.

Why Is Bun So Fast?

Engine: Unlike Node.js (which uses Google’s V8), Bun is powered by Apple’s JavaScriptCore (JSC)—the same engine behind Safari.
Language: Bun is implemented in Zig, a low-level language known for minimal overhead.
Startup Speed: Bun apps launch up to 4x faster than Node.js equivalents, with optimized memory usage and throughput.

What Makes Bun’s Ecosystem Stand Out?

Integrated Tooling: The bun command handles running scripts, installing dependencies, testing, and bundling—no more juggling npm, yarn, jest, or webpack.
Native TypeScript & JSX: Run .ts, .tsx, and .jsx files directly, thanks to Bun’s built-in transpiler—no need for tsc or Babel during development.
Module Compatibility: Bun supports both ES Modules (ESM) and CommonJS (CJS), making it easy to use the vast npm ecosystem.
Web Standard APIs: Familiar APIs like fetch, Request, Response, Headers, and Streams are globally available, improving code portability across environments.
Node.js Compatibility: Many Node.js core modules and global objects (fs, path, process, Buffer, etc.) are supported, allowing existing projects to migrate smoothly.

In short: Bun offers a single, fast executable for running, testing, building, and managing JavaScript/TypeScript projects—without the typical tooling sprawl.

Bun — A fast all-in-one JavaScript runtime. Bundle, install, and run JavaScript & TypeScript — all in Bun.

How to Install Bun: Quick Start for Every Platform

Getting started with Bun is straightforward. Here’s how to install it on your system:

Universal Command (macOS, Linux, WSL)

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

Installs Bun to ~/.bun/bin and updates your shell’s PATH.
Restart your terminal or run source ~/.zshrc (or your shell file) to activate.

Prefer to inspect the script first?

curl -fsSL https://bun.sh/install -o install.sh
bash install.sh

Alternative Installation Methods

npm:
```
npm install -g bun
```

Docker:

docker run --rm --init --ulimit memlock=-1:-1 oven/bun:latest bun --version

Homebrew (macOS):
```
brew tap oven-sh/bun
brew install bun
```
Windows:
Native support is experimental. Use WSL for best results or check the official docs for updates.

Verify Installation

bun --version

You should see the installed Bun version. Explore available commands with:

bun --help

First Steps: Running Your First Bun Script

Let’s launch a simple HTTP server using Bun’s integrated API.

Create a file called server.ts:

// server.ts
const server = Bun.serve({
  port: 3000,
  fetch(request: Request): Response {
    return new Response("Welcome to Bun!");
  },
});

console.log(`Listening on http://localhost:${server.port}`);

Bun.serve creates the server.
The fetch handler uses standard Web API objects.
Run with:
```
bun run server.ts
```
Visit http://localhost:3000 to see the server in action.

To stop the server, press Ctrl + C in your terminal.

Why is this different?
Bun handles both JavaScript and TypeScript out-of-the-box—no build step required. The API is familiar to anyone using modern web standards.

Native TypeScript and JSX Support in Bun

One of Bun’s most compelling features for API and backend developers is its seamless TypeScript and JSX support.

How Bun Handles TypeScript

No pre-compilation: Just run .ts or .tsx files directly.
Lightning-fast transpiler: Type annotations, interfaces, and enums are stripped on the fly.
JSX support: Use React-style JSX in .tsx/.jsx files—Bun’s transpiler handles it automatically.

Example:

// greet.ts
interface User { name: string; id: number; }

function greetUser(user: User) {
  console.log(`Hello, ${user.name} (ID: ${user.id})!`);
}

const user = { name: "Bun Developer", id: 123 };
greetUser(user);

console.log(`Bun version: ${Bun.version}`);

Run with:

bun run greet.ts

JSX Example:

// component.tsx
function MyComponent({ name }: { name: string }) {
  return <div className="greeting">Hello, {name}!</div>;
}

tsconfig.json Support

Bun respects tsconfig.json for JSX settings, import paths, and more. While Bun focuses on execution and quick transpilation (not full type-checking), it supports most config options relevant for development.

Bun’s Built-In APIs: Powerful Tools for API Developers

Beyond compatibility, Bun introduces its own high-performance APIs under the global Bun object.

Key Bun APIs

Bun.serve: Build ultra-fast HTTP and WebSocket servers (see later section).
Bun.file(path): Efficient, lazy file reading with multiple output options (.text(), .json(), .arrayBuffer(), .stream()).
Bun.write(path, data): Atomic, fast file writes.
Bun.build(options): Programmatic access to Bun’s esbuild-compatible bundler.
Bun.spawn / Bun.spawnSync: Run child processes, with async streaming or synchronous APIs.
Bun.Transpiler: Use Bun’s transpiler for TypeScript/JSX conversion programmatically.
Bun.password.hash / .verify: Secure, built-in password hashing (Argon2id, bcrypt).
Bun.env: Fast, global environment variable access.
Bun.version & Bun.revision: Runtime version and commit hash.
Bun.sleep, Bun.sleepSync: Pause execution for a given duration.
Bun.resolve / Bun.resolveSync: Module resolution utilities.

Why does this matter?
API developers can skip extra dependencies for common tasks, keeping their stacks lean and performant.

Web Standard APIs: Write Portable, Modern Code

Bun prioritizes implementing standard web APIs, making your server-side code portable and familiar.

Supported Web APIs

Fetch API: fetch, Request, Response, Headers
URL API: URL, URLSearchParams
Streams: ReadableStream, WritableStream, TransformStream
Encoding: TextEncoder, TextDecoder
Blob & File: For binary/file data
FormData: For handling form submissions and uploads
WebSocket: Fully compatible with browser and server APIs
Timers: setTimeout, setInterval, etc.
Console: Standard logging
Crypto: crypto.subtle, crypto.randomUUID(), crypto.getRandomValues()
Performance: performance.now()

Why Use Web Standards?

Portability: Move code between Bun, Node.js, Deno, browsers, and edge platforms with fewer changes.
Familiarity: Lower learning curve for frontend and backend developers alike.
Long-term stability: Built on WHATWG and W3C standards.

Building High-Performance HTTP Servers with Bun

The centerpiece for API and backend developers is Bun.serve—a blazing-fast, modern HTTP/WebSocket server.

Practical Example: Routing and JSON Handling

import { type Server } from "bun";

const server: Server = Bun.serve({
  port: 8080,
  hostname: "0.0.0.0",
  async fetch(req, server) {
    const url = new URL(req.url);

    if (url.pathname === "/") {
      return new Response("Homepage");
    }
    if (url.pathname === "/about") {
      return new Response("About Us page");
    }
    if (url.pathname === "/greet" && req.method === "GET") {
      const name = url.searchParams.get("name") || "World";
      return new Response(`Hello, ${name}!`);
    }
    if (url.pathname === "/data" && req.method === "POST") {
      try {
        const data = await req.json();
        return new Response(JSON.stringify({ received: data }), {
          headers: { 'Content-Type': 'application/json' },
        });
      } catch {
        return new Response("Invalid JSON body", { status: 400 });
      }
    }
    return new Response("Page Not Found", { status: 404 });
  },
  error(error) {
    console.error("[Server Error]", error);
    return new Response("Something went wrong!", { status: 500 });
  },
  development: true,
});

console.log(`Bun server listening on http://${server.hostname}:${server.port}`);

Highlights for API Teams:

Fully async, streaming-capable handlers
Standard Request/Response objects
Centralized error handling
Hot reloading with server.reload()
Minimal boilerplate for clean, testable code

Making HTTP Requests with Bun’s Fetch API

For outbound calls (consuming APIs, microservices, etc.), Bun’s global fetch is standards-aligned and extremely fast.

Example: GET, POST, and Timeout Handling

async function makeRequests() {
  const url = "https://httpbin.org";

  // GET request
  const getResponse = await fetch(`${url}/get?param1=value1`);
  const getData = await getResponse.json();
  console.log("GET Response:", getData.args);

  // POST JSON
  const postData = { name: "Bun", type: "Runtime" };
  const postResponse = await fetch(`${url}/post`, {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      "Accept": "application/json",
    },
    body: JSON.stringify(postData),
  });
  const postResult = await postResponse.json();
  console.log("POST Response:", postResult.json);

  // Request with timeout
  const controller = new AbortController();
  setTimeout(() => controller.abort(), 2000);
  try {
    await fetch(`${url}/delay/5`, { signal: controller.signal });
  } catch (error: any) {
    if (error.name === 'AbortError') {
      console.log("Fetch aborted due to timeout.");
    }
  }
}

await makeRequests();

API Testing Tip:
Use tools like Apidog to design and test API endpoints, then integrate with Bun’s fetch for production-ready code and end-to-end validation.

Real-Time APIs: WebSocket Support in Bun

Modern APIs often require real-time communication. Bun’s WebSocket support is built-in and adheres to browser standards on both client and server.

WebSocket Client Example

const ws = new WebSocket("wss://ws.postman-echo.com/raw");

ws.addEventListener("open", () => {
  ws.send("Hello from Bun client!");
});

ws.addEventListener("message", (event) => {
  console.log("Received:", event.data);
});

WebSocket Server with Pub/Sub

import { type ServerWebSocket, type WebSocketHandler } from "bun";

const wsHandler: WebSocketHandler<{ authToken: string }> = {
  open(ws) {
    ws.send("Welcome to the Bun WebSocket server!");
    ws.subscribe("the-group-chat");
    ws.publish("the-group-chat", `User joined.`);
  },
  message(ws, message) {
    server.publish("the-group-chat", `[User] ${message}`);
  },
  close(ws) {
    ws.unsubscribe("the-group-chat");
    server.publish("the-group-chat", `User left.`);
  },
};

const server = Bun.serve<{ authToken: string }>({
  port: 8081,
  fetch(req, server) {
    const url = new URL(req.url);
    if (url.pathname === "/ws") {
      return server.upgrade(req, { data: { authToken: "token" } }) ? undefined : new Response("WebSocket upgrade failed", { status: 400 });
    }
    return new Response("Not a WebSocket endpoint. Try /ws", { status: 404 });
  },
  websocket: wsHandler,
});

Pub/Sub built-in:
Easily manage group chats, notifications, or collaborative sessions across WebSocket clients.

Bringing It All Together—and Where Apidog Fits In

Bun empowers API and backend teams to:

Launch blazing-fast HTTP/WebSocket servers with minimal configuration
Run TypeScript and JSX natively—no build steps or extra tools
Use modern, portable APIs for maximum code reuse
Eliminate much of the tooling overhead typical in Node.js workflows

For API developers and teams:
While Bun streamlines runtime and server logic, tools like Apidog help you design, test, document, and collaborate on APIs efficiently. Apidog’s integrated platform bridges the gap between backend logic and team productivity, so your API workflow—from Bun code to live documentation—is seamless.

💡 Want to generate beautiful API documentation or supercharge your team’s API productivity? Apidog integrates design, testing, and team workflows—a streamlined Postman alternative.

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