react ref获取元素高度

import ReactDOM from "react-dom";

class App extends Component {
  constructor(props) {
    super(props);
    this.state = {
      width: 0
    };
  }

  componentDidMount() {
    const width = this.divElement.clientWidth;
    this.setState({ width });
  }

  render() {
    return (
      
 (this.divElement = divElement)}>
        Size: {this.state.width}px but it should be 18px after the render
      
    );
  }
}

const rootElement = document.getElementById("root");
ReactDOM.render(, rootElement);