Not able to display the current data sent from server - node.js

Question

i am using nodejs as server, I am sending parameters to mongodb and saving it in database after that i am fetching it from database. But when i try to fetch the data i can't able to see the current data in nodejs terminal ,but it will be present in database. Again if i send the other data i will able to see the previous data but not the current data which i have sent now. I think my server is calling find function before save function. what should i do to make my save function to complete its task and then it should call the find function.
this mongodb code
import mongoose from 'mongoose';
var Schema = mongoose.Schema;
//connect to a MongoDB database
var db = mongoose.connect('mongodb://127.0.0.1:27017/student');
mongoose.connect('connected', function() {
console.log("database connected successfully")
});
var userSchema = new Schema({
Name: {
type: String,
required: true
},
Age: {
type: Number,
required: true
}
}, {
collection: ('studentcollection2')
});
var User = mongoose.model('User', userSchema);
function createStudent(name, age) {
var list = new User({
Name: name,
Age: age
});
list.save(function(err) {
if (err) throw err;
console.log("SUCCESSFUL");
});
}
function listStudent() {
User.find({}, function(err, studentcollection2) {
if (err) throw err;
console.log(studentcollection2);
});
}
exports.createStudent = createStudent; //module.exports = User;
exports.listStudent = listStudent;
this is my server code
import config from './config';
import express from 'express';
const server = express();
import mongodbInterface from './mongodbInterface';
server.set('view engine', 'ejs');
server.get('/', (req, res) => {
res.render('index', {
content: '...'
})
});
console.log(typeof mongodbInterface.createStudent);
mongodbInterface.createStudent("a9", 112);
mongodbInterface.listStudent();
server.use(express.static('public'));
server.listen(config.port, () => {
console.info('express listening on port', config.port);
});

Answer 1

I haven't run your code, but I imagine it's coming down to these two lines:
mongodbInterface.createStudent("a9", 112);
mongodbInterface.listStudent();
Both of these statements call asynchronous functionality. That is to say that when you call createStudent("a9", 112);, that functionality will run asynchronously in the background, and Node will continue to call listStudent(); straight away. Therefore, your createStudent method may not have written the data to the database by the time your listStudent function is run.
To solve this, you need to make use of callback functions to only retrieve the data once it's actually been saved. For example, you could do the following:
function createStudent(name, age, cb) {
var list = new User({
Name: name,
Age: age
});
list.save(function(err) {
if (err) return cb(err);
return cb();
});
}
function listStudent(cb) {
User.find({}, function(err, studentcollection2) {
if (err) return cb(err);
return cb(null, studentCollection2);
});
}
Then, in your server code:
mongodbInterface.createStudent("a9", 112, function(err) {
if (err) throw err;
mongodbInterface.listStudent(function(err, students) {
console.log(students); // You should see the newly saved student here
});
});
I'd recommend reading more about Node.js callbacks here - they are really core to the usage of Node.js.

Answer 2

The mongo methods are async so the data will write whenever it's ready. Use a callback function, promise or async/await to wait for the result without blocking the event loop and control order of execution.