App.js

import React, {Component, useState} from 'react';

// Class
class App extends Component {
  constructor(props) {
    super(props);
    this.state = {counter: 0};
  }
  
  render() {
    return (
      <button onClick={() => this.setState({counter: this.state.counter + 1})}>
        {counter}
      </button>
    );
  }
}

// Functional
const App = () => {
  const [counter, setCounter] = useState(0);
  return (
    <button onClick={() => setCounter(counter + 1)}>{counter}</button>
  );
};