Express 4 Error: Can't set headers after they are sent

Question

I'm trying to set up a simple chat system using Express. I just migrated from v.3 to v.4 and now I get this error message:

   info  - socket.io started
Express server listening on port 3000
GET / 304 790.443 ms - -
Error: Can't set headers after they are sent.
    at ServerResponse.OutgoingMessage.setHeader (http.js:707:11)
    at ServerResponse.res.setHeader (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/connect/lib/patch.js:134:22)
    at Object.expressInit [as handle] (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/lib/middleware.js:20:42)
    at next (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/connect/lib/proto.js:174:15)
    at Object.query [as handle] (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/connect/lib/middleware/query.js:43:5)
    at next (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/connect/lib/proto.js:174:15)
    at Function.app.handle (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/connect/lib/proto.js:182:3)
    at Server.app (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/connect/lib/connect.js:67:37)
    at Manager.handleRequest (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/socket.io/lib/manager.js:564:28)
    at Server.<anonymous> (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/socket.io/lib/manager.js:118:10)
GET /javascripts/jquery.min.js 304 7.874 ms - -
GET /javascripts/jquery.min.js 304 7.966 ms - -
GET /javascripts/chat.js 304 4.132 ms - -
GET /javascripts/chat.js 304 4.213 ms - -
GET /socket.io/socket.io.js 404 11.137 ms - 35
GET /stylesheets/style.css 304 1.715 ms - -
GET /stylesheets/style.css 304 1.798 ms - -
Error: Can't set headers after they are sent.
    at SendStream.headersAlreadySent (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/send/index.js:319:13)
    at SendStream.send (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/send/index.js:501:17)
    at onstat (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/send/index.js:600:10)
    at Object.oncomplete (fs.js:107:15)
Error: Can't set headers after they are sent.
    at SendStream.headersAlreadySent (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/send/index.js:319:13)
    at SendStream.send (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/send/index.js:501:17)
    at onstat (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/send/index.js:600:10)
    at Object.oncomplete (fs.js:107:15)
Error: Can't set headers after they are sent.
    at SendStream.headersAlreadySent (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/send/index.js:319:13)
    at SendStream.send (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/send/index.js:501:17)
    at onstat (/Users/bruger/Google Drev/Uni/8. semester/Real-Time-Webapplikationer/0786OS_Code/Chapter 04/section02/awesome-chat/node_modules/express/node_modules/send/index.js:600:10)
    at Object.oncomplete (fs.js:107:15)
   debug - served static content /socket.io.js

When I first start up the server instance, everything seems fine, until I go to the browser, then the error comes.

I've been trying to solve the issue reading almost all of the posts about

Error: Can't set headers after they are sent.

But I can't figure out what I'm doing wrong? Heres my code where I set up the app.js file:

var express = require('express')
  , routes = require('./routes')
  , http = require('http')
  , path = require('path');

var methodOverride = require('method-override')                     
var json = require('json-middleware');                              
var multipart = require('connect-multiparty');                      
var multipartMiddleware = multipart();                              

var app = express();

// Express 4 version
var env = process.env.NODE_ENV || 'development';
if ('development' == env) {
   // configure stuff here
  app.set('port', process.env.PORT || 3000);
  app.set('views', __dirname + '/views');
  app.set('view engine', 'jade');
  app.use(express.favicon());
  app.use(express.logger('dev'));
  app.use(json);                                                    
  app.use('/public/javascripts/chat.js', multipartMiddleware);      
  //app.use(express.bodyParser());                                  
  //app.use(express.methodOverride());                              
  app.use(methodOverride('_method'))
  app.use(app.router);
  app.use(express.static(path.join(__dirname, 'public')));
}


app.get('/', routes.index);

var server = http.createServer(app).listen(app.get('port'), function(){
  console.log("Express server listening on port " + app.get('port'));
});

//routes.sockets.connect(server);
require('./routes/sockets.js').initialize(server);

It might be that the error is generated in my index.js or socket.js file, so I add these too. Index.js:

exports.index = function(req, res){
  res.render('index', { title: 'Express Chat' });
};

sockets.js:

var io = require('socket.io');

exports.initialize = function(server) {
  io = io.listen(server);
  io.sockets.on("connection", function(socket){
    socket.on('message', function(message){
      message= JSON.parse(message);
      if(message.type == "userMessage"){
        socket.get('nickname', function(err, nickname){
          message.username=nickname;
          socket.broadcast.send(JSON.stringify(message));
          message.type = "myMessage";
          socket.send(JSON.stringify(message));
        });        
      }
    });

    socket.on("set_name", function(data){
      socket.set('nickname', data.name, function(){
        socket.emit('name_set', data);
    socket.send(JSON.stringify({type:'serverMessage', message: 'Welcome to the most interesting chat room on earth!'}));          
      });
    });
  });
}

I'm new to express and node.js and been trying to get my head around it for two days now, so all help would be highly appreciated!

Answer 1

This kind of error usually stems from not return -ing your send functions and then something later in the execution also sends data.

To mitigate this you should always return sends.

  return socket.send(JSON.stringify(message));

This will ensure that multiple headers aren't set.