Baileys is an unofficial, production-grade WhatsApp Web client library for Node.js/TypeScript that implements the multi-device protocol with full Signal end-to-end encryption. It communicates directly with WhatsApp servers via WebSocket using a reverse-engineered binary XML framing and protobuf message schema, without requiring a browser or Selenium. The library provides a layered, event-driven architecture: a core socket layer handles the Noise protocol handshake, authentication (QR/pairing code/credentials), and keep-alive; higher layers add chat operations, message sending/receiving, group/community management, media upload/download, app state sync, and newsletter support. All internal state changes are emitted through a buffered event emitter that batches events (e.g., `messages.upsert`, `chats.upsert`, `connection.update`) for efficient consumer processing. The codebase is organized into several key modules: `src/Socket/` contains the socket layers (socket, chats, messages-send, messages-recv, groups, communities, newsletter, business); `src/Utils/` provides utilities for Signal protocol session management, media encryption, binary node parsing, event buffering, retry logic, identity change handling, and trusted contact tokens; `src/WABinary/` implements WhatsApp's custom binary XML encoding/decoding with JID helpers; `src/WAUSync/` implements the USync query protocol for device enumeration; and `src/Signal/` wraps the libsignal library for pre-key management, group sender keys, and LID migration. The library depends on external packages such as `libsignal`, `ws`, `protobufjs`, `@hapi/boom`, `pino`, `lru-cache`, `async-mutex`, and optional media processing libraries (`sharp`, `jimp`, `link-preview-js`). Baileys is designed for integration into larger applications (bots, automation, analytics) and exposes a comprehensive API for sending/receiving messages, managing groups, syncing chat history, handling media, and reacting to real-time events. It includes extensive unit and end-to-end tests, a protobuf extraction tool (`proto-extract/`) that scrapes WhatsApp Web's JavaScript to regenerate the `.proto` schema, and a version update script that fetches the latest WhatsApp Web version. The library is actively maintained with frequent commits, CI/CD pipelines, and a clear migration guide for breaking changes.