bun

Bun is a fast all-in-one JavaScript runtime.

weekly downloads822K

versionv1.3.9

licenseMIT

categorytoolchain

bunbun.jsnodenode.jsruntimebundlertranspilertypescript

Overview

Bun is a modern JavaScript runtime built from scratch using Zig and JavaScriptCore (WebKit's engine) that aims to replace the entire Node.js ecosystem toolchain with a single, optimized executable. Instead of piecing together Node.js, npm, TypeScript, Jest, and webpack separately, Bun consolidates runtime execution, package management, bundling, and testing into one cohesive toolkit.

The project emerged from frustrations with JavaScript tooling fragmentation and performance bottlenecks. While Node.js revolutionized server-side JavaScript, its ecosystem evolved into a complex web of dependencies, build tools, and configuration files. Bun addresses this by providing a drop-in replacement that executes JavaScript and TypeScript natively, installs packages up to 30x faster, and starts processes 4x quicker than Node.js—all while maintaining compatibility with existing npm packages and Node.js APIs.

Bun targets developers building web servers, CLI tools, build scripts, and full-stack applications who want faster iteration cycles and simplified tooling. Companies adopting Bun report significantly reduced CI/CD times and simplified development environments. The runtime is production-ready for many use cases, though it's still maturing compared to Node.js's 14+ years of ecosystem development. With over 800,000 weekly downloads, Bun represents a generational shift in JavaScript infrastructure, particularly appealing to greenfield projects and teams willing to trade ecosystem maturity for raw performance and developer experience improvements.

Quick Start

typescript

import { serve } from 'bun';
import { readFile } from 'fs/promises';

const server = serve({
  port: 3000,
  async fetch(req) {
    const url = new URL(req.url);
    
    if (url.pathname === '/') {
      return new Response('Bun Server Running', {
        headers: { 'Content-Type': 'text/plain' }
      });
    }
    
    if (url.pathname === '/api/data') {
      const data = await readFile('./data.json', 'utf-8');
      return new Response(data, {
        headers: { 'Content-Type': 'application/json' }
      });
    }
    
    return new Response('Not Found', { status: 404 });
  }
});

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

// Run with: bun run server.ts
// Hot reload: bun --hot run server.ts
// Install packages: bun add express
// Run tests: bun test

Use Cases

High-throughput web APIs: Bun's optimized HTTP server handles 4x more requests per second than Node.js equivalents, making it ideal for REST APIs, GraphQL servers, and microservices where response latency directly impacts user experience. The native fetch API and WebSocket support simplify building real-time applications.

Monorepo development: Teams managing workspaces with shared dependencies benefit from Bun's package manager, which uses hardlinks instead of duplicating node_modules across projects. Installation times drop from minutes to seconds, dramatically improving developer onboarding and CI pipeline speed.

TypeScript-first applications: Projects written entirely in TypeScript can eliminate build steps by running .ts files directly with Bun. This removes the need for ts-node, tsx, or separate tsc compilation, streamlining development for backend services, CLI tools, and automation scripts.

Rapid prototyping and scripts: Developers building one-off scripts, data processing pipelines, or DevOps automation tools appreciate Bun's fast startup time and built-in TypeScript support. Running bun run script.ts executes immediately without configuration overhead.

Replacing Node.js in CI/CD: Build pipelines that run tests, lint code, and install dependencies see 50-70% time reductions by switching to Bun's test runner and package manager, cutting costs on GitHub Actions, GitLab CI, or CircleCI compute minutes.

Pros & Cons

Pros

+Dramatically faster performance: 4x faster startup, 25-30x faster package installs, significantly higher HTTP throughput compared to Node.js
+Native TypeScript and JSX support without transpilation configuration—run .ts, .tsx, and .jsx files directly
+All-in-one toolchain eliminates dependency hell and version conflicts between runtime, bundler, test runner, and package manager
+Drop-in Node.js compatibility with npm packages, built-in modules (fs, path, net), and familiar CLI commands
+Hot reload mode preserves application state and connections during development, unlike Node.js's full process restarts

Cons

−Ecosystem maturity gap: some npm packages with native addons or complex Node.js internals may have compatibility issues
−Smaller community and fewer Stack Overflow answers compared to Node.js's 14-year ecosystem
−Bundler still in beta with limited plugin ecosystem compared to webpack, Rollup, or esbuild
−Less battle-tested in production environments—enterprises may hesitate without long-term stability track record
−Advanced build tooling (custom Babel transforms, complex Webpack loaders) may not have Bun equivalents yet

Alternatives

esbuildv0.27.3

An extremely fast JavaScript and CSS bundler and minifier.

build-tool

105.3M / week

tsxv4.21.0

TypeScript Execute (tsx): Node.js enhanced with esbuild to run TypeScript & ESM files

toolchaincliruntime

23.9M / week

Comparisons

typescript vs bun

toolchain

Install

bash

npm install bun

bash

pnpm add bun

bash

bun add bun

Overview

Quick Start

typescript

import { serve } from 'bun';
import { readFile } from 'fs/promises';

const server = serve({
  port: 3000,
  async fetch(req) {
    const url = new URL(req.url);
    
    if (url.pathname === '/') {
      return new Response('Bun Server Running', {
        headers: { 'Content-Type': 'text/plain' }
      });
    }
    
    if (url.pathname === '/api/data') {
      const data = await readFile('./data.json', 'utf-8');
      return new Response(data, {
        headers: { 'Content-Type': 'application/json' }
      });
    }
    
    return new Response('Not Found', { status: 404 });
  }
});

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

// Run with: bun run server.ts
// Hot reload: bun --hot run server.ts
// Install packages: bun add express
// Run tests: bun test

Use Cases

Pros & Cons

Pros

+Dramatically faster performance: 4x faster startup, 25-30x faster package installs, significantly higher HTTP throughput compared to Node.js
+Native TypeScript and JSX support without transpilation configuration—run .ts, .tsx, and .jsx files directly
+All-in-one toolchain eliminates dependency hell and version conflicts between runtime, bundler, test runner, and package manager
+Drop-in Node.js compatibility with npm packages, built-in modules (fs, path, net), and familiar CLI commands
+Hot reload mode preserves application state and connections during development, unlike Node.js's full process restarts

Cons

−Ecosystem maturity gap: some npm packages with native addons or complex Node.js internals may have compatibility issues
−Smaller community and fewer Stack Overflow answers compared to Node.js's 14-year ecosystem
−Bundler still in beta with limited plugin ecosystem compared to webpack, Rollup, or esbuild
−Less battle-tested in production environments—enterprises may hesitate without long-term stability track record
−Advanced build tooling (custom Babel transforms, complex Webpack loaders) may not have Bun equivalents yet