Creating a singleton both in plain JS and using node commonJS cache behavior https://app.pluralsight.com/player?course=javascript-practical-design-patterns&author=jonathan-mills&name=javascript-practical-design-patterns-m3&clip=14&mode=live

main.js

var taskHandler = require('./taskHandler');
var myrepo = require('./Repo');

myrepo.save('fromMain');
myrepo.save('fromMain');
myrepo.save('fromMain');
taskHandler.save();
taskHandler.save();
taskHandler.save();
taskHandler.save();

Repo.js

var repo = function(){
    var called = 0;

    var save = function (task){
        called++;
        console.log('Saving ' + task +
                    ' Called ' + called + ' times');
    }

    console.log('newing up task repo');
    return {
        save: save
    }
}
//Using repo() or new repo makes Node use the cache behavior to create a new repository only if it doesn't exist
module.exports = repo();

singleton.js

var TaskRepo = (function () {
    var taskRepo;
    function createRepo() {
        var taskRepo = new Object("Task");
        return taskRepo;
    }
    return {
        getInstance: function () {
            if (!taskRepo) {
                taskRepo = createRepo();
            }
            return taskRepo;
        }
    };
})();

var repo1 = TaskRepo.getInstance();
var repo2 = TaskRepo.getInstance();

if(repo1 === repo2){
    console.log("Same TaskRepo") 
}

taskHandler.js

var myrepo = require('./Repo');


var taskHandler = function () {
    return {
        save: function (){
            myrepo.save('Hi from taskHandler');
        }
    }
}

module.exports = taskHandler();