Data does not display on first try only upon refresh - node.js

Question

I am new to node, express and mongo so I need some guidance. I set up a mongo database with 3 objects. I created an application with a route so that http://localhost:3000/Employeeid will lead the page to display all the 3 objects in the database, which the page does. However for each subsequent refresh, the same data gets displayed multiple number of times based on the number of refresh. (i.e. For the first refresh, the data duplicates once. For the second refresh, the data duplicates twice.) Does anyone know what may be wrong?
var express = require('express')
var app = express()
var MongoClient = require('mongodb').MongoClient
var url = 'mongodb://localhost:27017'
var str = ''
app.route('/Employeeid').get(function(req, res) {
MongoClient.connect(url, { useNewUrlParser: true }, function(err, client) {
var db = client.db('EmployeeDB')
var cursor = db.collection('Employee').find()
cursor.forEach(function(item) {
if (item != null) {
str = str + ' Employee id&nbsp ' +
item.Employeeid + '</br>'
}
})
res.send(str)
client.close()
})
})
var server = app.listen(3000, function() {})

Answer 1

You are appending to str every time the /EmployeeId route is called. To fix this, move str inside the callback:
app.route('/Employeeid').get(function(req, res) {
MongoClient.connect(url, { useNewUrlParser: true }, function(err, client) {
var str = ''
var db = client.db('EmployeeDB')
var cursor = db.collection('Employee').find()
cursor.forEach(function(item) {
if (item != null) {
str = str + ' Employee id&nbsp ' +
item.Employeeid + '</br>'
}
})
res.send(str)
client.close()
})
})
Side note - you should reuse the Mongo connection instead of calling connect and close on every request.

Answer 2

The Reason why your getting duplicate results is you are not making str = null after sending the result. you are concatenating str for each request because str is a global variable. your code can be modified for performance as like this.
var express = require('express')
var app = express()
var MongoClient = require('mongodb').MongoClient
var url = 'mongodb://localhost:27017'
var str = ''
var db = null
MongoClient.connect(url, { useNewUrlParser: true }, function(err, client) {
if( err ) {
throw new Error( err );
}
db = client.db('EmployeeDB') // we are storing db reference in global variable db
});
app.route('/Employeeid').get(function(req, res) {
db.collection('Employee').find().toArray( function(err, cursor){
if( err ) {
res.send('');
throw new Error(err );
}
cursor.forEach(function(item) {
if (item != null) {
str = str + ' Employee id&nbsp ' +
item.Employeeid + '</br>'
}
}
res.send(str)
str = '';
})
})
})
var server = app.listen(3000, function() {})
If you don't move res.send() into callback function you may send result before getting data from db due to async nature of node.