addItem = event => {
  event.preventDefault();
  const {name, price} = this.state;
  const itemsInState = this.state.items;
  const itemsArrayLength = itemsInState.length;
  const id = itemsArrayLength
             ?
             (itemsInState[itemsArrayLength - 1].id + 1)
             :
             1;
  this.setState({
   items: [
     ...itemsInState,
     Object.assign({}, {
       id,
       name,
       price
     })
   ],
   name: "",
   price: ""
 })
};