版本：3.0.0-rc4
- https://cdn.bootcdn.net/ajax/libs/socket.io/3.0.0-rc4/socket.io.js
- https://cdn.bootcdn.net/ajax/libs/socket.io/3.0.0-rc4/socket.io.min.js
- https://cdn.bootcdn.net/ajax/libs/socket.io/3.0.0-rc4/socket.io.msgpack.min.js
socket.io 是用于构建实时通讯服务的框架。