Button Component

Button.js

class Button extends React.Component {
state = { counter: 0};

handleClick = () => {
 this.setState((prevState) => ({
    counter: prevState.counter + 1
 }));
};

render() {
  return (
  <button onClick={this.handleClick}>
  {this.state.counter}
  </button>
  );
 }
}

ReactDOM.render(<Button />, mountNode);

React JS File.