Send a JSON array as response from Node.js

Question

I am fetching data from a MongoDB database then putting it in a cursor to send that as a Node.js response.

var router = express.Router();
var MongoClient = require('mongodb').MongoClient;
var url = 'mongodb://localhost/EmployeeDB';
/* GET users listing. */
router.get('/', function(req, res, next) {
  //res.send('respond with a resource');
  MongoClient.connect(url, function(err, db) {
    var cursor = db.collection('Employee').find();
    cursor.each(function(err, doc) {

      console.log(doc);
      arrayres = doc ;
     res.send(doc);

    });
    db.close();
  });
});

module.exports = router;

It sends only the first record then I get this error:

Error [ERR_HTTP_HEADERS_SENT]: Cannot remove headers after they are sent to the client
    at ServerResponse.removeHeader (_http_outgoing.js:528:11)
    at ServerResponse.send

Notice: I get this error only when there are multiple records to send as response.

You can only send back one response to the browser (be it `res.send()`, `res.end()`, `res.sendFile()`, `res.json()` or any other). You can't have that inside a `.forEach()`. — Jeremy Thille, Feb 28 '20 at 14:34
With Mongoose, you'd be like `Model.find().lean().exec( docs => res.json(docs))` — Jeremy Thille, Feb 28 '20 at 14:38

Abel Vinuela · Answer 1 · 2020-02-28T15:17:16.573

1

You are sending the response twice. Which is impossible ( look at Why can't we do multiple response.send in Express.js? )

  res.send('respond with a resource');

Here and

res.send(arrayres);

Here.

Here is a working example based on jeremy's answer :

router.get('/', function (req, res, next) {
    MongoClient.connect(url, function (err, db) {
        var cursor = db.collection('Employee').find();

        let employees = []
        const pushData = async () => {
            cursor.forEeach( function (doc) {
                employees.push(doc);
            });
        }
        const sendResponse = async () => {
            await pushData();
            res.json(employees);
        }
    });
});

edited Feb 28 '20 at 15:17

answered Feb 28 '20 at 14:27

Abel Vinuela

13
4

still having the same error even after sending a single response , please watch the post content update , thanks . – yassine bengharsallah Feb 28 '20 at 14:34
This is still the same issue, you are sending a response for each employee you have. You must send one and only one response. – Abel Vinuela Feb 28 '20 at 14:36
Look at the edit i made. I hope it will work for you – Abel Vinuela Feb 28 '20 at 15:18

Jeremy Thille · Answer 2 · 2020-02-28T14:51:08.320

0

You can only send back one response to the browser (be it res.send(), res.end(), res.sendFile(), res.json() or any other). So, you can't have that inside a .forEach().

First, build an array, then send your data back once.

router.get('/', function (req, res, next) {
    MongoClient.connect(url, function (err, db) {
        var cursor = db.collection('Employee').find();

        let employees = []

        cursor.forEeach( function (doc) {
            employees.push(doc);
        });

        res.json(employees);
    });
});

Or with Mongoose :

Employee.find().lean().exec( (err,docs) => res.json(docs))

edited Feb 28 '20 at 14:51

answered Feb 28 '20 at 14:42

Jeremy Thille

25,196
9
41
59

thank you @jeremy Thille for your answer but i actually sending an empty array "[ ]" i think there is a problem with push into the array – yassine bengharsallah Feb 28 '20 at 14:48
Yes, it should be `cursor.forEeach( function (doc)`. Fixed – Jeremy Thille Feb 28 '20 at 14:51
still having the same issue ! empty array sent to the browser . – yassine bengharsallah Feb 28 '20 at 15:07
Well that's a different problem. How to build the data you send back is something else. I don't use Mongo's native driver, so I don't know how to use `cursor`. Mongoose makes your life so much easier :) Anyway you get the idea, you can't use `res.end()` more than once. – Jeremy Thille Feb 28 '20 at 15:25

Send a JSON array as response from Node.js

2 Answers2