Example for constructor based object creation

object-constructor.js

function Product( initialStock ) {
  let stock = initialStock;
  
  /* ... extra fields omitted for simplicity ... */
  
  this.updateStock = function( newStock ) {
    stock += newStock;
  }
  this.getStock = function( ) {
    return stock;
  }
};

const product = new Product( 4 );
console.log( product.stock ); // undefined!
console.log( product.getStock( ) ); // 4
product.updateStock( 3 );
console.log( product.getStock( ) ); // 7

const product2 = new Product( 0 );
console.log( product2.getStock( ) ); // 0