ویرایش شده 

  case "ADD_TO_BASKET":
      const productId = action.payload.productID;
      const payloadCount = action.payload.count;
      if (
        state.basket.findIndex((product) => product.productID === productId) !==
        -1
      ) {
        const basket = state.basket.reduce((basketAcc, product) => {
          if (product.productID === productId) {
            basketAcc.push({
              ...product,
              count: product.count + payloadCount,
            });
          } else {
            basketAcc.push(product);
          }
          return basketAcc;
        }, []);
        newState = { ...state, basket };
      } else {
        newState = {
          ...state,
          basket: [...state.basket, { ...action.payload }],
        };
      }
      break;

فایل Details.js مربوط به single product قسمت input

 <input
            type="number"
            className="form-control form-qty mr-2 w-25"
            placeholder={1}
            onChange={(e) => {
              const { value } = e.target;
              setItemCount(parseInt(value));
            }}
          />