Operation `users.insertOne()` buffering timed out after 10000ms MongooseError

Question

I am simply getting this error while trying to connect to my mongodb database. My code is very simple as i am only testing mongoose out. Here is the the code:

index.js

let mongoose = require('mongoose');
let User = require('./User');

mongoose.connect('mongodb://localhost/testdb');

let user = new User({ name:'Karl', age:69 });

user.save().then(() => console.log('User Saved'));

User.js

let mongoose = require('mongoose');

let userSchema = new mongoose.Schema({
    name: String,
    age: Number
})

module.exports = mongoose.model('User', userSchema);