Unable to close mongoose connection - mongodb

Question

I am unable to close mongoose connection when I am storing mongoose.connection in a variable called db and creating a separate method to closing the connection via variable db I have tried both method db.close & mongoose.disconnect()
const mongoose = require('mongoose');
var db;
const connectToDB = function (callback) {
let dbUrl;
if (environmentTokens.enviroment === "test") {
dbUrl = localDBUrl;
} else {
dbUrl = environmentTokens.mongoDBUrl;
}
mongoose.connect(dbUrl);
db = mongoose.connection
db.on('error', (err) => {
tracer.error('Connection error with database', err);
})
db.on('connected', () => {
tracer.info('Connected with database', dbUrl);
console.log('Mongoose default connection connected');
callback();
})
// db.on('disconnected', function () {
// console.log('Mongoose default connection disconnected');
// });
// mongoose.connection.close(function () {
// console.log('Mongoose default connection disconnected through app termination')})
};
const getDB = function () {
return db;
};
const disconnectDB = function () {
db.close(function () {
console.log('Mongoose default connection disconnected through app termination')})
// mongoose.disconnect(function () {
// console.log('Mongoose default connection disconnected through app termination')})
}
module.exports = {
connectToDB, disconnectDB, getDB
};
And calling the disconnectDB method from index.js file
process.on('SIGINT', () => {
disconnectDB();
process.exit()
})
I tried to close the connection using following method
mongoose.disconnect(function () {
console.log('Mongoose default connection disconnected');
});
mongoose.connection.close(function () {
console.log('Mongoose default connection disconnected through app termination')})
within disconnectDB method but non of them is closing the connection but when i am using these method inside connectToDB then i am able to close the connection. DOn't understand why this is behaving like this