class People extends Component {
  constructor(props) {
    super(props);
    this.state = {
      people:[{ name: "mary", age: 25 }, { name: "john", age: 30 }]
    };
  }

  updateJohnAge = ()=>{
    const people = [...this.state.people];
    people[1].age=31;
    this.setState({people});
    //This will not work.
    //People is a new array, but john is the same object
  }

  render() {
    return (
      <div>
        <Person person={this.state.people[1]}></Person>
        <button onClick={this.updateJohnAge}>Set john age to 31</button>
      </div>
    );
  }
}