freeCodeCamp/guide/english/javascript/singleton-in-javascript/index.md

title
Creating a Singleton in JavaScript

Creating a Singleton in JavaScript

A singleton is an object that only allows one instance of itself to be created and allows a global point of access to it. The singleton design pattern can be useful if you have some data that should remain the same throughout your application.

Let's Get Started

Let's create an object with an immediately-invoked function expression (or IIFE, pronounced "iffy") that will give us the effect of a singleton.

var singletonFn = (function(){ // Created IIFE Function
  var dataCounter = 0;
  return { // Any code inside this return stuff will be accessible directly using objectname.
    
    getDataCounter: function(){
      return dataCounter;
    },
  
    setDataCounter: function(val){
      dataCounter = val;
    },
    
    fishNames: ["SimpleFish"] // Can create variables, arrays, etc.
  }
})();

console.log(singletonFn.getDataCounter()); // 0 by default

singletonFn.setDataCounter(20);

console.log(singletonFn.getDataCounter()); // 20

console.log(singletonFn.fishNames); // ["SimpleFish"]

Here's another example of a singleton:

var Person;

(function() {
  let instance;

  Person = function(fName, lName, job) {
    // constructor
    this.fName = fName || 'John';
    this.lName = lName || 'Doe';
    this.job = job || 'Software Engineer';
    
    if (!instance) {
      instance = this;
    }

    this.getFullName = function () {
      return this.fName + ' ' + this.lName;
    }
    
    this.getJob = function () {
      return this.job;
    }

    this.print = function () {
      console.log(this.getFullName() + ' ' + this.getJob());
    }

    return instance;
  }
})();

//initialize these "guys" but they point to the same object
var guy1 = new Person();
var guy2 = new Person();

// proof that they're the same instance
guy1.print(); // "John Doe Software Engineer"
guy2.print(); // "John Doe Software Engineer"
console.log(guy1 == guy2); //true

// modify guy2 and see that guy1 has changed as well
guy2.fName = 'Jane';
guy2.lName = 'Doo';
guy2.job = 'Project Manager';
guy1.print(); // "Jane Doo Project Manager"
guy2.print(); // "Jane Doo Project Manager"