If there is only going to be one instance of an object, should I still use a constructor? [closed]

Question

I've been traveling deeper into the JS world and came across 3 different ways I could develop the frontend cart of a website:

Constructor with Prototype Functions

var cart = function(){
    this.items = {}
}

cart.prototype.increaseItemQty = function(partNumber){
    if(this.items[partNumber]){
        this.items[partNumber].qty += 1;
    } else {
        this.items[partNumber] = {
            qty : 1
        };
    }
}

cart = new cart();

Methods Within the Constructor

var cart2 = function(){
    this.items = {};
    this.increaseItemQty = function (partNumber) {
        if(this.items[partNumber]){
            this.items[partNumber].qty += 1;
        } else {
            this.items[partNumber] = {
                qty : 1
            };
        }
    }
}

cart2 = new cart2();

Object Methods

var cart3 = {
    items : {},
    increaseItemQty : function(partNumber){
        if(this.items[partNumber]){
            this.items[partNumber].qty += 1;
        } else {
            this.items[partNumber] = {
                qty : 1
            };
        }
    }
};

If I know that there will only be one instance of the cart, then should I just use the Object Method method? Is there any reason why I should still use a constructor?

Answer 1

Yes, there is a (minor) reason you shouldn't use a constructor: the instance will hold a reference to the constructor function via instance.[[Prototype]].constructor, and thus it won't be garbage collected. It will waste memory uselessly because you won't instantiate it again.

var cart = new function(){ /* ... */ };
console.log(cart.constructor); // Won't be garbage-collected

If you prefer a function approach you can still do something like the following. This way you can also have private variables.

var cart = function() {
  // You can add private variables here
  this.items = {};
  this.increaseItemQty = function(partNumber) {
    if(this.items[partNumber]) {
      this.items[partNumber].qty += 1;
    } else {
      this.items[partNumber] = {
        qty : 1
      };
    }
  };
  return this;
}.call({});
cart.increaseItemQty(0);
console.log(cart.items);