const numbers = [1, 2, 3, 4, 5];

const listItems = numbers.map((numbers) =>

  <li>{numbers}</li>

);

ReactDOM.render(

  <ul>{listItems}</ul>,

  document.getElementById('example')

);

function ListItem(props) {

  // 对啦！这里不需要指定key:

  return <li>{props.value}</li>;

}

function NumberList(props) {

  const numbers = props.numbers;

  const listItems = numbers.map((number) =>

    // 又对啦！key应该在数组的上下文中被指定

    <ListItem key={number.toString()}

              value={number} />

  );

  return (

    <ul>

      {listItems}

    </ul>

  );

}

const numbers = [1, 2, 3, 4, 5];

ReactDOM.render(

  <NumberList numbers={numbers} />,

  document.getElementById('example')

);