Bun

Fast all-in-one JavaScript runtime and toolkit for building server-side applications. Combines a runtime, package manager, test runner, and bundler into a single toolchain built on JavaScriptCore. Supports JavaScript, TypeScript, and JSX out of the box with zero configuration, and aims for drop-in compatibility with Node.js projects.

Trusted by teams at Midjourney for WebSocket-powered notification pipelines, Claude Code for CLI tooling, and Railway for serverless functions. Bun differentiates by bundling four tools that typically require separate setup: use bun install as a faster npm alternative, bun test as a Jest-compatible test runner, and bun build for bundling, or adopt the full runtime for greenfield apps.

Key capabilities:

• Runtime with built-in HTTP server, WebSocket support, and native PostgreSQL, MySQL, SQLite, Redis, and S3 APIs
• Package manager up to 30x faster than npm with workspaces, isolated installs, and dependency auditing
• Jest-compatible test runner with snapshot testing, watch mode, and built-in code coverage
• Bundler for TypeScript, JSX, React, and CSS with single-file executable output and cross-compilation
• TypeScript, JSX, YAML, and CSS imports without config, plus hot reload for development

Backend developers use it for APIs and real-time services where startup time and throughput matter. Frontend teams run bun ./index.html for instant HMR with React or other frameworks. Script authors replace Node for faster execution and simpler TypeScript support. The bun:ffi API enables calling C libraries directly, and Bun.$ provides cross-platform shell scripting from JavaScript.