How can i resolve 'Cannot set headers after they are sent to the client'

Question

How can I resolve Cannot set headers after they are sent to the client:

app.js

var express = require('express');
var session = require('express-session');
var mongoose = require('mongoose');
var app = express();
var ejs = require('ejs');
var port = 3000;
var bodyParser = require('body-parser');
var mongoDB = "mongodb://localhost:27017/vinavdb";

app.set('views', __dirname + '/admin') app.engine('html', ejs.renderFile);
app.set('view engine', 'ejs');

app.set('trust proxy', 1);
app.use(session({
  secret: 'dsghbrtdfhbdfg64545TRYFFHGGJNN',
  resave: false,
  saveUninitialized: true
}))

app.use(bodyParser.json());
app.use(bodyParser.urlencoded({
  extended: true
}));

var sess;

mongoose.Promise = global.Promise;
mongoose.connect(mongoDB, {
  useNewUrlParser: true
});

var nameSchema = new mongoose.Schema({
  firstname: String,
  lastname: String,
  email: String,
  password: String
});
var User = mongoose.model("User", nameSchema);

app.get("/", (req, res) => {
  sess = req.session;
  if (sess.email) {
    res.redirect("/admin");
  } else {
    res.sendFile(__dirname + "/index.html");
  }

});

app.get("/login", (req, res) => {
  res.sendFile(__dirname + "/login.html");
});
app.post("/addname", function(req, res) {
  var myData = new User(req.body);
  User.findOne({
    email: req.body.email
  }, function(err, resv) {
    if (resv == null) {
      myData.save().then(item => {
        res.send("Name saved to database")
      }).catch(err => {
        res.status(400).send("Unable to save to database");
      });
      res.send("ThankYou For your Registration")
    } else if (resv.email == req.body.email) {
      res.send("Email is already registered");
    } else {
      res.send("Srry data is not allowed");
    }
  });
});

app.post("/login", function(req, res, next) {
  User.findOne({
    email: req.body.email
  }, function(err, vals) {
    if (vals == null) {
      res.end("Invalid Logins");
    } else if (vals.email == req.body.email && vals.password == req.body.password) {
      sess = req.session;
      sess.email = req.body.email;
      res.redirect('/admin');
    } else {
      res.send("Srry data is not allowed");
    }
  });
});
app.route('/admin').get(function(req, res, next) {
  sess = req.session;
  if (sess.email) {
    res.send(__dirname + "/admin/index.html");
  } else {
    res.write('Please login first.');
  }
})

app.listen(port, () => {
  console.log("Server listening on port " + port);
});

Answer 1

Cause of your error : You are trying to send a response of a single request twice.

One request have one response.

Once response is send, you cannot send it again for the same request.In your /addname API, You are trying to send response twice. So remove one.

Here .save() is asynchronous function so node will not wait and execute res.send("ThankYou For your Registration") first and later once record will be saved it will try to send res.send("Name saved to database") so you are getting error here.

app.post("/addname", function(req, res) {
  var myData = new User(req.body);
  User.findOne({
    email: req.body.email
  }, function(err, resv) {
    if (resv == null) {
      myData.save().then(item => {
              console.log("Name saved to database")
              res.send("ThankYou For your Registration")
      }).catch(err => {
        res.status(400).send("Unable to save to database");
      });
    } else if (resv.email == req.body.email) {
      res.send("Email is already registered");
    } else {
      res.send("Srry data is not allowed");
    }
  });
});

Answer 2

After you execute a res.send you cannot call it again in the same request, it only allows one response per request. In your code, I think in this part it can happens:

if (resv == null) {
      myData.save().then(item => {
        res.send("Name saved to database")
      }).catch(err => {
        res.status(400).send("Unable to save to database");
      });
      res.send("ThankYou For your Registration")
    }

In this if, when you are saving myData you are sending a response, but asynchronously you already sent another response previously ("ThankYou For your Registration").

Hope it helps