Older browsers don't support WebSockets. So in order to provide a similar experience, we need to fall back to various browser/plugin-specific techniques to emulate WebSocket functionality to the best of the deprecated browser's ability.
Naturally, this is a mine field, requiring hours of browser testing and in some cases highly specific knowledge of proprietary protocols (for example, IE's
Active X htmlfile object).
socket.io provides a WebSocket-like API to the server and client to create the best-case real-time experience across a wide variety of browsers, including old (IE 5.5+) and mobile (iOS Safari, Android) browsers.
On top of this, it also provides convenience features, such as disconnection discovery ...