How to be sure that message via socket.io has been received to the client?

Question

How to check that message sent with socket.io library has been received to the client. Is there special method for it in socket.io?

Thanks for you

Answer 1

You should use the callback parameter while defining the event handler.

A typical implementation would be as follows:

Client side

var socket = io.connect('http://localhost');
socket.emit('set', 'is_it_ok', function (response) {
    console.log(response);
});

Server side

io.sockets.on('connection', function (socket) {
    socket.on('set', function (status, callback) {
        console.log(status);
        callback('ok');
    });
});

Now check the console on the server side. It should display 'is_it_ok'. Next check console on client side. It should display 'ok'. That's the confirmation message.

Update

A socket.io connection is essentially persistent. The following in-built functions let you take action based on the state of the connection.

socket.on('disconnect', function() {} ); // wait for reconnect
socket.on('reconnect', function() {} ); // connection restored  
socket.on('reconnecting', function(nextRetry) {} ); //trying to reconnect
socket.on('reconnect_failed', function() { console.log("Reconnect failed"); });

Using the callback option shown above is effectively a combination of the following two steps:

socket.emit('callback', 'ok') // happens immediately

and on the client side

socket.on('callback', function(data) {
    console.log(data);
});

So you don't need to use a timer. The callback runs immediately except if the connection has any of the following states - 'disconnect', 'reconnecting', 'reconnect_failed'.