## TL;DR

The WebSocket API is the browser-side interface to the WebSocket protocol (RFC 6455), providing full-duplex, bidirectional communication between browser and server over a single TCP connection.

## Core Explanation

Usage: `const ws = new WebSocket('wss://example.com')`. Events: `onopen`, `onmessage`, `onerror`, `onclose`. Methods: `send()`, `close()`. The `readyState` property tracks connection state (CONNECTING=0, OPEN=1, CLOSING=2, CLOSED=3). Binary data can be sent as Blob or ArrayBuffer. WebSocket connections start as HTTP (upgrade) and are not HTTP after the handshake.

## Further Reading

- [The WebSocket API (W3C)](https://websockets.spec.whatwg.org/)