node.js error - Can't sent headers after they are sent

Question

I'm using the ebay-api for node.js and when I refresh the page for some reason I get an error in the console:

can't send headers after they are sent

Here is my code, can anyone help me understand why I am getting the error when refreshing the page?

// example simple request to FindingService:findItemsByKeywords

var ebay = require('../index.js');
var http = require('http');

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


app.set('port', process.env.PORT || 5000);

app.get('/getEbay', function (req, res) {
    console.log('inside get');
    //  for avoiding crossbrowser-error
    res.header('Access-Control-Allow-Origin', '*');
    res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
    res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization');
    res.header('Content-Type', 'application/json');
    var params = {};

    params.keywords = ["cat"];
    params['paginationInput.entriesPerPage'] = 10;
    ebay.ebayApiGetRequest({
        serviceName: 'FindingService',
        opType: 'findItemsByKeywords',
        appId: 'MYAPPID',      // FILL IN YOUR OWN APP KEY, GET ONE HERE: https://publisher.ebaypartnernetwork.com/PublisherToolsAPI
        params: params,
        // filters: filters,
        parser: ebay.parseItemsFromResponse    // (default)
    },
    // gets all the items together in a merged array
  function ebayApiGetRequest(error, items) {
      if (error) throw error;

      console.log('Found', items.length, 'items');
      //  res.send(items);
        console.log(JSON.stringify(items));


      res.contentType('application/json');

      res.send(JSON.stringify(items));

      //  }  
  }
);

});


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

console.log('Listening on port 5000...');

I think that my mistake may be in the order for calling the functions or maybe there is a way to avoid this wrong calling?

Answer 1

well guys we figured out the problem !

Here's the new code that is working :

// example simple request to FindingService:findItemsByKeywords
var http = require('http');

var express = require('express');
var app = express();
var io = require('socket.io');
var ebay = require('../index.js');


app.set('port', process.env.PORT || 5000);

app.get('/getEbay', function(req, res) {
    console.log('inside get');
    //  for avoiding crossbrowser-error
    res.header('Access-Control-Allow-Origin', '*');
    res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
    res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization');
    res.header('Content-Type', 'application/json');

    getEbaybyResults(function(error, items) {
          if (error) throw error;

          console.log('Found', items.length, 'items');
          //  res.send(items);
          console.log(JSON.stringify(items));


        // res.contentType('application/json');
        //  console.log(try);
       // res.send(JSON.stringify(items));
        res.end(JSON.stringify(items));
    });


});

function getEbaybyResults(callback) {

   var params = {};

    params.keywords = ["cat"];
    params['paginationInput.entriesPerPage'] = 10;
    ebay.ebayApiGetRequest(
      {
        serviceName: 'FindingService',
        opType: 'findItemsByKeywords',
        appId: 'MYAPPID',      // FILL IN YOUR OWN APP KEY, GET ONE HERE: https://publisher.ebaypartnernetwork.com/PublisherToolsAPI
        params: params,
        // filters: filters,
        parser: ebay.parseItemsFromResponse    // (default)
      },
      function(error, items) {
        callback(error, items);
      }
    );
    //ebayApiGetRequest();
}

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

//console.log('Listening on port 5000...');

Answer 2

Instead of this:

res.send(JSON.stringify(items));
res.send(items);

Try for:

res.end(JSON.stringify(items));

Answer 3

function ebayApiGetRequest(error, items) {
      if (error) throw error;

      console.log('Found', items.length, 'items');
      //  res.send(items);
        console.log(JSON.stringify(items));


      res.contentType('application/json');

      res.send(JSON.stringify(items));// you send here
      res.send(items);//and you send again right afterwards?

      //  }  
  }

If you actually execute send twice, then of course you're going to send the data twice. One of those res.send() is enough.