MongoDB connection close error

mongodb_practice.js

var MongoClient = require('mongodb').MongoClient;

MongoClient.connect('mongodb://localhost:27017/course', function(err, db) {
    
    if(err) throw err;
    
    for (var i = 0; i<10; i++) {

    	var fields = ["Engineering",  "Medicine", "Marketing"];
    	var langs = ["EN", "POR", "ES"];

    	var doc = {user: i, field: fields[Math.round(Math.random()*2)], language: langs[Math.round(Math.random()*2)] }

    	db.collection('students').insert(doc, function(err, inserted) {
	        if(err) {
	        	console.log(err.message);
	        } 
	        
	        return db.close(); // Everything is being inserted OK but I still receiving a "Connection Close by Application" error. WHY?
    	});

    }

});