Before we get to memoization we need to talk about caching. Like most concepts in programming we can use a box analogy. Caching in simplest terms is putting data in an easily accessible box to speed up our code by keeping a fast record of it on hand. Memoization is a special way of caching that is commonly used in dynamic programming and in our case React. To state it more clearly memoization is a caching a return value (data in our box) from a past function to solve a problem so you don't have to calculate it again.

It is but one of the many optimization techniques out there. In the beginning it may not seem like a big deal but as functions grow in size and complexity, their calculations can be more expensive to run. A code snippet for demonstration purposes:

const cache = {}; 
const factorial = n => {

    if (cache[n]) {
        return cache[n];
    }
    let value;
    if (n === 0) value = 1;
    else value = (factorial(n - 1) * n);
    cache[n] = value;
    return value;
};

When it comes to React Web Applications, we can look at through the lens of the example of a dropdown menu component. Using libraries like 'reselect' we can avoid re-rendering the component if the value of the state doesn't change.

Though in this case it's a bit misleading because in my items cart redux is running a shallow equality check to see if the values were the same to try to save us a re-render. We're memoizing the transformation logic being ran through our 'mapStateToProps' with creating selectors and createStructuredSelector from 'reselect's library though. As you can see with some of the example code snippet here:

const CartDropdown = ({ cartItems }) => (
  <div className="cart__dropdown">
    <div className="cart__items">
      {cartItems.length ? (
        cartItems.map((cartItem) => (
          <CartItem key={cartItem.id} item={cartItem} />
        ))
      ) : (
        <span className="empty__message">Your cart is empty</span>
      )}
    </div>
    <CustomButton>GO TO CHECKOUT</CustomButton>
  </div>
);
const mapStateToProps = createStructuredSelector({
  cartItems: selectCartItems,
});
export default connect(mapStateToProps)(CartDropdown);

It is something to keep in mind as you grow in your developing journey as it can cut down tremendously on your code refactoring.

If you want to take a look at the whole project to get a better high level idea you can see it Here on my Github