Socket.io with promise or async callbacks

Question

This might be kind of an anti pattern, but what would be the best way to simulate a a promise callback when emitting data from client to server using socket.io?

For some event I would very much like it to behave like a normal get request so the client send data to the server and the server could reply with a response and then resolve or reject the promise.

Answer 1

Here's what I did. I created a function that emits a socket command, and then returns a promise. My one caveat is I haven't done a lot with this code yet, so it might need some tweaking. It also requires Q for its promises/deferreds.

Client function definition:

var EmitPromise = function( socket, command, data ) {

    var deferred = Q.defer();

    socket.emit(command, data, function( response ) {

        if( typeof response === "object" ) {

            if( response.success === true ) {

                deferred.resolve(response.data);

            } else {
                if( typeof response.message === "string" ) {
                    deferred.reject( response.message );
                } else {
                    deferred.reject( "The request was not successful." )
                }
            }
        } else {

            deferred.reject( "The response to your request could not be parsed." );
        }

    });

    return deferred.promise.timeout( 30000, "The request took too long to respond." );
}

Client side code to make the call:

EmitPromise( socket, "getValue", "username" )
.then(
    function( data ) {

        console.log(data);
        return EmitPromise( socket, "getValue", "anotherValue" );

    }, function( message ) {
        console.log(message);
    }
).then(
    //Chain your commands from here
);

Server side handler

The most important part of this handler is that the second parameter, here named setValueResult must be called with an object that contains a "success" key that holds a true or false value. This was a decision on my part to provide a way to reject the promise in case of some kind of error on the server side.

socket.on( 'getValue', function( valueName, setValueResult ) {

    var value = getValue(valueName) //Do something with value here

    if( value ) {
        setValueResult({
            success : success,
            data : value
        });
    } else {
        setValueResult({
            success : success
            message : "Unable to retrieve value"
        });
    }
}