sketch_message.js

    var remote_down = false;

    socket.on('message', function(data) {

        switch (data.act) {
            case "down":
                remote_down = true;
                ctx.strokeStyle = data.color;
                ctx.beginPath();
                ctx.moveTo(data.x, data.y);
            case "move":
                console.log("remote: " + data.x, data.y);
                ctx.lineTo(data.x, data.y);
                ctx.stroke();
            case "up":
                if (!remote_down) return;
                ctx.lineTo(data.x, data.y);
                ctx.stroke();
                ctx.closePath();
                remote_down = false;
        }   
    });