react.dev/examples/reconciliation/no-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 = {
      list: [
        { id: toDoCounter, createdAt: date },
      ],
      toDoCounter: toDoCounter
    }
  }

  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=id</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><th>created at</th>
          </tr>
          {
            this.state.list.map((todo, index) => (
              <ToDo
                key={todo.id}
                {...todo}
              />
            ))
          }
        </table>
      </div>
    )
  }
}

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