How to send response to front end after execution of mongodb request? - node.js

Question

I am creating sign up page. Where, I first check whether user email already present in our mongodb database or not. If it is present then I want to send error message to frontEnd. However, I am failing to do that, I think it might be because of asynchronous behavior of JavaScript.My Code is as following:
var myObj , myJSON
var SignUpUserEmail, SignUpUserPassword, SignUpUserName, SignUpErr
http.createServer(function (req, res) {
res.writeHead(200, {'Content-Type': 'text/html'});
var q = url.parse(req.url, true).query
SignUpUserEmail = q.SignUpUserEmail
SignUpUserPassword = q.SignUpUserPassword
SignUpUserName = q.SignUpUserName
MongoClient.connect("mongodb://localhost:27017/ABC",function(err,
database) {
if (err) throw err;
var db=database.db('ABC')
let findOneParam = {"UserEmail":SignUpUserEmail}
db.collection('Profiles').findOne(findOneParam, function(err, result) {
if (err) throw err;
if(!result) {
db.collection('Profiles', function(err, collection){
if (err) throw err;
collection.insertOne({"UserId":"ProfileA0001",
"UserEmail":SignUpUserEmail,
"UserPassword":SignUpUserPassword,
"UserName":SignUpUserName,
"IsEmailAuthenticated":"false"
}, function(err, res){
if (err) throw err;
SignUpErr = "document inserted"
console.log("SignUpErr inside:", SignUpErr)
})
})
} else {
SignUpErr = "Email already has been registered."
console.log("SignUpErr inside:", SignUpErr)
}
})
})
console.log("SignUpErr outside:", SignUpErr)
myObj = {"SignUpErr":SignUpErr};
myJSON = JSON.stringify(myObj);
res.end(myJSON);
}).listen(9000);
Note: "SignUpErr inside:" giving correct result. however, "SignUpErr outside:" shows it as undefined.

Answer 1

Note: "SignUpErr inside:" giving correct result. however, "SignUpErr outside:" shows it as undefined.
This is because of the asynchronous nature of the nodejs. SignUpErr will be undefined until the time it is initialized within the db.collection('Profiles',function(){}) call.
So, to fix this, you need to send response within db.collection('Profiles',function(){}). that's, after the initilization.
Making those changes to your code,
'use strict';
const http = require('http');
http.createServer(function (req, res) {
res.statusCode = 200; // Setting the status code
res.setHeader('Content-Type', 'text/plain'); // Setting the content-type for response
let {SignUpUserEmail, SignUpUserPassword, SignUpUserName} = url.parse(req.url, true).query;
MongoClient.connect("mongodb://localhost:27017/ABC", function (err, database) {
if (err) {
throw err;
}
let db = database.db('ABC');
db.collection('Profiles').findOne({
UserEmail: SignUpUserEmail
}, function (err, result) {
if (err) {
throw err
}
if (result) {
let msg = "Email already has been registered.";
console.log("SignUpErr inside:", msg);
return res.end(JSON.stringify({
SignUpErr: "document inserted"
}));
}
db.collection('Profiles', function (err, collection) {
if (err) throw err;
collection.insertOne({
"UserId": "ProfileA0001",
"UserEmail": SignUpUserEmail,
"UserPassword": SignUpUserPassword,
"UserName": SignUpUserName,
"IsEmailAuthenticated": "false"
}, function (err, dbresult) {
if (err) {
throw err;
}
let msg = "document inserted";
console.log("SignUpErr inside:", msg);
return res.end(JSON.stringify({
SignUpErr: "document inserted"
}));
})
});
});
});
}).listen(9000);

Answer 2

I usually use express as my web framework which comes with res.send() method, where you can send your response . I usually build a JSON response, and send it as res.send(JSON.stringify(data)); There is also res.JSON(data).
If you wish to use HTTP module , then you can use res.end() method.
Details are provided here . Hope this helps.