process.nextTick(function() { throw err; }); Error: Can't set headers after they are sent

Question

I'm trying get data from MongoDB in a Node.js file.
I'm getting this error:

/home/jay/node_project/user_data_manag/node_modules/mongodb/lib/utils.js:98
process.nextTick(function() { throw err; });

Error: Can't set headers after they are sent.
at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:335:11)
at ServerResponse.header (/home/jay/node_project/user_data_manag/node_modules/express/lib/response.js:719:10)
at ServerResponse.json (/home/jay/node_project/user_data_manag/node_modules/express/lib/response.js:247:10)
at ServerResponse.send (/home/jay/node_project/user_data_manag/node_modules/express/lib/response.js:152:21)
at /home/jay/node_project/user_data_manag/routes/api.js:42:13
at /home/jay/node_project/user_data_manag/node_modules/mongodb/lib/cursor.js:777:7
at handleCallback (/home/jay/node_project/user_data_manag/node_modules/mongodb/lib/utils.js:96:12)
at /home/jay/node_project/user_data_manag/node_modules/mongodb/lib/cursor.js:741:16
at handleCallback (/home/jay/node_project/user_data_manag/node_modules/mongodb/lib/utils.js:96:12)
at /home/jay/node_project/user_data_manag/node_modules/mongodb/lib/cursor.js:675:5

// Dependencies
var express = require('express');
var router = express.Router();
var mongo = require('mongodb').MongoClient;
var objectId = require('mongodb').ObjectID;
var assert = require('assert');

var url = 'mongodb://localhost/rest_test';


router.post('/user', function(req, res, next) {
  if (req.body.name == "Login") {
    var result = [];
    mongo.connect(url, function(err, db) {
      var data = db.collection('products').find();
      data.forEach(function(doc, err) {
        result.push(doc);
      }, function() {
        db.close();
        res.send({
          iteamd: result
        });
      });
    });
  } else {
    res.send("data");
  }
  res.end();
});

//insert data
router.post('/put-data', function(req, res, next) {

  var item = {
    title: req.body.name,
    content: req.body.sku,
    author: req.body.price
  };
  var data = req.body.name;

  mongo.connect(url, function(err, db) {
    var cursor = db.collection('products').find({
      "name": data
    });
    res.send(cursor.toString);
  });

});


// Return router
module.exports = router;

Answer 1

Don't call res.send() and res.end() in the same request handler.

res.send() will call res.end() for you ( source ). res.end() is typically used after calling multiple res.write() .

Answer 2

If /user POST is called multiple times and the POST n doesn't close the connection with db.close(); before the n+1 POST you could get the same error.

Just solved on my script.