React

HTML

<div id="app"></div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

li {
  margin: 8px 0;
}

h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

React

class InputApp extends React.Component {
	static getDerivedStateFromProps(nextProps, state) {
  	alert('state changed')
    return null;
  }
   
  constructor(props) {
    super(props)
    this.state = {
    	value: null,
    }
    this.onChange = this.onChange.bind(this);
  }
  
  onChange(event) {
  	this.setState({ value: event.target.value });
  }
  
  render() {
    return (
      <div>
        <h2>Input:</h2>
        <input type="text" onChange={this.onChange}/> 
      </div>
    )
  }
}

ReactDOM.render(<InputApp />, document.querySelector("#app"))