How to wait for a WebSocket's readyState to change

Question

I\'m trying to implement a WebSocket with a fallback to polling. If the WebSocket connection succeeds, readyState becomes 1, but if it fails, readyState

Answer 1

Just like you defined an onmessage handler, you can also define an onerror handler. This one will be called when the connection fails.

var socket = new WebSocket(url);
socket.onmessage = onmsg;
socket.onerror = function(error) {
    // connection failed - try polling
}

Answer 2

Here is a more elaborate explanation. First off, check the specific browser API, as not all browsers will be on the latest RFC. You can consult the

You don't want to run a loop to constantly check the readystate, it's extra overhead you don't need. A better approach is to understand all of the events relevant to a readystate change, and then wire them up appropriately. They are as follows:

onclose An event listener to be called when the WebSocket connection's readyState changes to CLOSED. The listener receives a CloseEvent named "close".

onerror An event listener to be called when an error occurs. This is a simple event named "error".

onmessage An event listener to be called when a message is received from the server. The listener receives a MessageEvent named "message".

onopen An event listener to be called when the WebSocket connection's readyState changes to OPEN; this indicates that the connection is ready to send and receive data. The event is a simple one with the name "open".

JS is entirely event driven, so you need to just wire up all of these events and check for the readystate, this way you can switch from WS to polling accordingly.

I recommend you look at the Mozilla reference, it's easier to read than the RFC document and it will give you a good overview of the API and how it works (link).

Don't forget to do a callback for a retry if you have a failure and poll until the callback for a successful reconnect is fired.

Answer 3

This is simple and it work perfectly... you can add condition about maximal time, or number of try to make it more robust...

function sendMessage(msg){
    // Wait until the state of the socket is not ready and send the message when it is...
    waitForSocketConnection(ws, function(){
        console.log("message sent!!!");
        ws.send(msg);
    });
}

// Make the function wait until the connection is made...
function waitForSocketConnection(socket, callback){
    setTimeout(
        function () {
            if (socket.readyState === 1) {
                console.log("Connection is made")
                if (callback != null){
                    callback();
                }
            } else {
                console.log("wait for connection...")
                waitForSocketConnection(socket, callback);
            }

        }, 5); // wait 5 milisecond for the connection...
}