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What Socket.IO is

The bidirectional channel between the Socket.IO server (Node.js) and the Socket.IO client (browser, Node.js, or another programming language) is established with a WebSocket connection whenever possible, and will use HTTP long-polling as fallback.

Socket.IO is now completely instrumented by a minimalistic yet tremendously powerful utility called debug by TJ Holowaychuk.

You will find below some code examples with common testing libraries:

Common/known issues:

Starting with v3, Socket.IO now has first class support for TypeScript.