title: React Redux: Should reducers contain any logic
body: <p>I have a cart reducer function with add, update and delete cases. I also have a product array within the redux store.
   When there are two items added to the product array, instead of having two items, I increment the quantity value. My main question is, should the reducers include any logic i.e. determine if the products array already contains the exact product and just returns an update on quantity of product or should this behavior be handled within the presentational component checking for existing products and either adding a new product or updating the quantity?</p>
  
  <pre><code>function CartReducer (state = initialState, action) {
    switch (action.type) {
      case AddToCart:
        return {
          ...state,
          products: [...state.products, action.product],
          totalPrice: state.totalPrice += (action.price * action.quantity)
        }
  
      case RemoveItemCart:
  
        return {
          ...state,
          products: [
            ...state.products.slice(0, action.index),
            ...state.products.slice(action.index + 1)
          ]
        }
  
      case UpdateItemQuantity:
        return {
          ...state,
          products: state.products.map((product, index) =&gt; {
            if (index === action.index) {
              return Object.assign({}, product, {
                quantity: action.quantity
              })
            }
            return product
          })
        }
  
      default:
        return state
    }
  }
  </code></pre>