react.dev/examples/reconciliation/index-used-as-key.js

const ToDo = props => (
  <tr>
    <td>
      <label>{props.id}</label>
    </td>
    <td>
      <input />
    </td>
    <td>
      <label>
        {props.createdAt.toTimeString()}
      </label>
    </td>
  </tr>
);

class ToDoList extends React.Component {
  constructor() {
    super();
    const date = new Date();
    const todoCounter = 1;
    this.state = {
      todoCounter: todoCounter,
      list: [
        {
          id: todoCounter,
          createdAt: date,
        },
      ],
    };
  }

  sortByEarliest() {
    const sortedList = this.state.list.sort(
      (a, b) => {
        return (
          a.createdAt - b.createdAt
        );
      }
    );
    this.setState({
      list: [...sortedList],
    });
  }

  sortByLatest() {
    const sortedList = this.state.list.sort(
      (a, b) => {
        return (
          b.createdAt - a.createdAt
        );
      }
    );
    this.setState({
      list: [...sortedList],
    });
  }

  addToEnd() {
    const date = new Date();
    const nextId =
      this.state.todoCounter + 1;
    const newList = [
      ...this.state.list,
      {id: nextId, createdAt: date},
    ];
    this.setState({
      list: newList,
      todoCounter: nextId,
    });
  }

  addToStart() {
    const date = new Date();
    const nextId =
      this.state.todoCounter + 1;
    const newList = [
      {id: nextId, createdAt: date},
      ...this.state.list,
    ];
    this.setState({
      list: newList,
      todoCounter: nextId,
    });
  }

  render() {
    return (
      <div>
        <code>key=index</code>
        <br />
        <button
          onClick={this.addToStart.bind(
            this
          )}>
          Add New to Start
        </button>
        <button
          onClick={this.addToEnd.bind(
            this
          )}>
          Add New to End
        </button>
        <button
          onClick={this.sortByEarliest.bind(
            this
          )}>
          Sort by Earliest
        </button>
        <button
          onClick={this.sortByLatest.bind(
            this
          )}>
          Sort by Latest
        </button>
        <table>
          <tr>
            <th>ID</th>
            <th />
            <th>created at</th>
          </tr>
          {this.state.list.map(
            (todo, index) => (
              <ToDo
                key={index}
                {...todo}
              />
            )
          )}
        </table>
      </div>
    );
  }
}

ReactDOM.render(
  <ToDoList />,
  document.getElementById('root')
);