无法在react js中正确使用Refs，无法读取未定义的属性

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.myRef = React.createRef();
  }

  componentDidMount() {
    // 此时可以安全地访问this.myRef.current
    console.log(this.myRef.current);
  }

  render() {
    return <div ref={this.myRef}>Hello, World!</div>;
  }
}

import React, { useRef } from 'react';

function MyComponent() {
  const myRef = useRef(null);

  useEffect(() => {
    // 此时可以安全地访问myRef.current
    console.log(myRef.current);
  }, []);

  return <div ref={myRef}>Hello, World!</div>;
}

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.myRef = null;
  }

  render() {
    return <div ref={(el) => (this.myRef = el)}>Hello, World!</div>;
  }
}