The WebSocket API - The Definitive Guide to HTML5 WebSocket - page 19

WebSocket Event: Error
The error event fires in response to unexpected failures. The corresponding callback to
the error event is called onerror . Errors also cause WebSocket connections to close. If you
receive an error event, you can expect a close event to follow shortly. The code and reason in
the close event can sometimes tell you what caused the error. The error event handler is
a good place to call your reconnection logic to the server and handle the exceptions coming
from the WebSocket object. Listing 2-8 shows an example of how to listen for error events.
Listing 2-8. Sample Error Event Handler
// Event handler for errors in the WebSocket object
ws.onerror = function(e) {
console.log("WebSocket Error: " , e);
//Custom function for handling errors
handleErrors(e);
};
WebSocket Event: Close
The close event fires when the WebSocket connection is closed. The corresponding
callback to the close event is called onclose . Once the connection is closed, the client
and server can no longer receive or send messages.
Not ■ The WebSocket specification also defines ping and pong frames that can be used
for keep-alive, heartbeats, network status probing, latency instrumentation, and so forth, but
the WebSocket API does not currently expose these features. Although the browser receives
a ping frame, it will not fire a visible ping event on the corresponding WebSocket. Instead,
the browser will respond automatically with a pong frame. However, a browser-initiated ping
that is unanswered by a pong after some period of time may also trigger the connection
close event. Chapter 8 covers WebSocket pings and pongs in more detail.
You also trigger the onclose event handler when you call the close() method and
terminate the connection with the server, as shown in Listing 2-9.
Listing 2-9. Sample Close Event Handler
// Event handler for closed connections
ws.onclose = function(e) {
console.log("Connection closed", e);
};
The WebSocket close event is triggered when the connection is closed, which can
be due to a number of reasons such as a connection failure or a successful WebSocket
closing handshake. The WebSocket object attribute readyState reflects the status of the
connection (2 for closing or 3 for closed).