App.jsx

import React from 'react'

class App extends React.Component {
  state = {
    items: ["Item 1", "Item 2", "Item 3", "Item 4", "Item 5", "Item 6"]
  };

  addItem = newItem => {
    this.setState({ items: [...this.state.items, newItem] });
  };

  render() {
    return (
      <div>
        <ItemList items={this.state.items} />
        <NewItem onAdd={this.addItem} />
      </div>
    );
  }
}