1class Example extends React.Component {
2  constructor(props) {
3    super(props);
4    this.state = {
5      count: 0
6    };
7  }
8
9  render() {
10    return (
11      <div>
12        <p>You clicked {this.state.count} times</p>
13        <button onClick={() => this.setState({ count: this.state.count + 1 })}>
14          Click me
15        </button>
16      </div>
17    );
18  }
19}

1class BalanceInquiry extends Component {
2  constructor(props) {
3    super(props);
4    this.state = {
5      totalIncome: 0,
6    };
7  }
8
9  render() {
10    return <div>totalIncome {this.state.totalIncome}</div>;
11  }
12}
13

1const [stateName, SetStateName] = useState();
2SetStateName('any name');

1  <button onClick={() => this.setState({ count: this.state.count + 1 })}>
2    Click me
3  </button>

1// State is essentially a global class variable
2// that is modified when the component updates
3
4class Clock extends React.Component {
5  constructor(props) {
6    super(props);
7    this.state = {date: new Date()};
8  }
9
10  render() {
11    return (
12      <div>
13        <h1>Hello, world!</h1>
14        <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
15      </div>
16    );
17  }
18}

1this.setState({
2    someState: obj
3}, () => {
4    this.afterSetStateFinished();
5});