The WebSocket technology has been standardized for over a decade and is now one of the main technologies powering the modern web, enabling event-driven, full-duplex communication between client and server with minimum overhead and low latency. WebSockets are ideal for building interactive, realtime digital experiences that provide delightful user experiences. The WebSocket Handbook is a resource for developers who want to learn about this technology and how to use it to build event-driven, realtime apps. It covers the core building blocks of the WebSocket technology, its characteristics, and advantages, as well as providing step-by-step walkthroughs on how to build realtime web apps with WebSockets and Node.js.