title: Make variable sum of two other variables after change
body: <p>Please look at the following code. There is an issue in it which I am struggling to understand:</p>
  
  <pre><code>let sideCount = 0;
  let mealCount = 0;
  let itemCount = sideCount + mealCount;
  
  const pizzaOrder = ({size, crustType, topping, quantity}) =&gt; {
      quantity = Math.round(parseInt(quantity));
      mealCount += quantity;
      if (topping === "") {
          console.log(`Order: ${quantity} ${size.toTitleCase()} ${crustType.toLowerCase()} crust plain cheese pizza`);
      } else {
          console.log(`Order: ${quantity} ${size.toTitleCase()} ${crustType.toLowerCase()} crust ${topping} pizza`);
      }
      console.log(`Meal Count after ordering ${quantity} pizzas: ${mealCount}`);
      console.log(`Current item count: ${itemCount}`);        
  };
  </code></pre>
  
  <p>(There's more, but this is what is important for the question)</p>
  
  <p>I have looked all over StackOverflow for referencing and pass by value or pass by reference, but I couldn't find anything that solved my problem. </p>
  
  <p>When I update <code>mealCount</code>, it doesn't change the value of <code>itemCount</code>. How do I fix this?</p>
  
  <pre><code>Order: 10 Medium thin crust pepperoni pizza 
  Meal Count after ordering 10 pizzas: 10 
  Current item count: 0 
  Order: 3 burgers with Relish 
  Meal Count after ordering burger: 13 
  Current item count: 0 
  Meal Count: 0 Side Count: 0 
  Meal Count: 0 
  Side Count: 0 
  Meal Count: 0 
  Side Count: 0 
  Your total is $0.00. Enjoy your meal! 
  </code></pre>