react中的portal是做什么的

const dogRoot = document.getElementById("dog-house");
const catRoot = document.getElementById("cat-house");

class Puppy extends React.Component {
  constructor(props) {
    super(props);
    // 创建一个容器标签
    this.el = document.createElement("div");
  }

  componentDidMount() {
  	// 把容器标签挂到 catRoot DOM下
    catRoot.append(this.el);
  }

  componentWillUnmount() {
    catRoot.removeChild(this.el);
  }

  render() {
  	// 利用portal把Puppy的内容放到容器里
    return ReactDOM.createPortal(this.props.children, this.el);
  }
}

class Dog extends React.Component {
  constructor(props) {
    super(props);
    this.state = { bark: 0 };
    this.handleClick = this.handleClick.bind(this);
  }

  handleClick() {
  	// 点击的时候 bark + 1
    this.setState((state) => ({
      bark: state.bark + 1,
    }));
  }

  render() {
 	// 看上去Puppy组件是在Dog挂在Dog组件里，但其实它被挂载在其它地方
    return (
      <div onClick={this.handleClick}>
        <p>The number of times a big dog barks: {this.state.bark}</p>
        <h3>Dog: </h3>
        <p>I can't see my children, but I can feel them</p>
        <Puppy>
          <Bully target={'Puppy'}/>
        </Puppy>
        <Bully target={'Dog'}/>
      </div>
    );
  }
}

ReactDOM.render(<Dog />, dogRoot);

function Bully(props) {
  return (
    <>
      <button>Bully the {props.target}</button>
    </>
  );
}