Products.jsx

    addItem(product){
      // Get current list of products
      let products = this.state.products
      let idx = this.state.products.indexOf(product)
      
      // Update the total price by quantity * price of the added product
      let totalPrice = this.state.totalPrice + (product.price * product.quantity)
      if (this.state.products.indexOf(product) !== -1) {
          products[idx].quantity += 1;
      } else {
          products.push(product);
      }
      
      // Update the state
      this.setState({
        products: products,
        totalPrice: totalPrice,
        checkoutDisabled: this.totalPriceAboveMinimumOrder(totalPrice)
      })
    }

    removeItem(product){
      let idx = this.state.products.indexOf(product)
      let products = this.state.products
      let totalPrice = this.state.totalPrice - (product.price * product.quantity)
      
      // Remove single item if multiple items added, else remove the whole item from cart
      if (this.state.products[idx].quantity > 1 ){
          products[idx].quantity -= 1
      } else {
          products.splice(idx, 1)
      }
      this.setState({
          products: products,
          totalPrice: totalPrice,
          checkoutDisabled: this.totalPriceAboveMinimumOrder(totalPrice)}
        )
    }