JavaScript: The Definitive Guide, 6th Edition

Web Sockets

Chapter 18 showed how client-side JavaScript code can communicate over the network. The examples in that chapter all used HTTP, which means that they were all constrained by the fundamental nature of the HTTP: it is a stateless protocol that consists of client requests and server responses. HTTP is actually a fairly specialized network protocol. More general network connections over the Internet (or over local intranets) often involve longer-lived connections and bidirectional message exchange over TCP sockets. It is not safe to give untrusted client-side JavaScript code access to low-level TCP sockets, but the WebSocket API defines a secure alternative: it allows client-side code to create bidirectional socket-type connections to servers that support the WebSocket protocol. This makes it much easier to perform certain kinds of networking tasks.

The WebSocket Protocol

In order to use WebSockets from JavaScript, you only need to understand the client-side WebSocket API described here. There is no equivalent server-side API for writing WebSocket servers, but this section does include a simple server example that uses Node (Asynchronous I/O with Node) along with a third-party WebSocket server library. The client and server communicate over a long-lived TCP socket following rules defined by the WebSocket protocol. The details of the protocol are not relevant here, but it is worth noting that the WebSocket protocol is carefully designed so that web servers can easily handle ...