1class ClickCounter extends React.Component {
2  constructor(props) {
3    super(props);
4    this.state = { count: 0 };
5    this.handleClick = this.handleClick.bind(this);
6  }
7
8  handleClick() {
9    this.setState(state => {
10      return { count: state.count + 1 };
11    });
12  }
13
14  render() {
15    return [
16      <button key="1" onClick={this.handleClick}>
17        Update counter
18      </button>,
19      <span key="2">{this.state.count}</span>
20    ];
21  }
22}
How React Fiber works.
This is a StoryTime rendition of excellent blog post by Max Koretskyi. 
Click play or go step by step using the icons on top right.