webSocket的readyState属性用来定义连接状态，该属性的值有下面几种：
0 ：对应常量CONNECTING (numeric value 0)，
正在建立连接连接，还没有完成。The connection has not yet been established.
1 ：对应常量OPEN (numeric value 1)，
连接成功建立，可以进行通信。The WebSocket connection is established and communication is possible.
2 ：对应常量CLOSING (numeric value 2)
连接正在进行关闭握手，即将关闭。The connection is going through the closing handshake.
3 : 对应常量CLOSED (numeric value 3)
连接已经关闭或者根本没有建立。The connection has been closed or could not be opened.