React报错解决之ref返回undefined或null

import {useRef, useEffect} from 'react';

export default function App() {
  const ref = useRef();

  console.log(ref.current); // ?️ undefined here

  useEffect(() => {
    const el2 = ref.current;
    console.log(el2); // ?️ element here
  }, []);

  return (
    <div>
      <div ref={ref}>
        <h2>Hello</h2>
      </div>
    </div>
  );
}

import {useRef, useEffect} from 'react';

export default function App() {
  const ref = useRef();

  console.log(ref.current); // ?️ undefined here

  useEffect(() => {
    const el2 = ref.current;
    console.log(el2); // ?️ element here
  }, []);

  const handleClick = () => {
    console.log(ref.current); // ?️ element here
  };

  return (
    <div>
      <div ref={ref}>
        <h2>Hello</h2>
      </div>

      <button onClick={handleClick}>Click</button>
    </div>
  );
}

class CustomTextInput extends React.Component {
  constructor(props) {
    super(props);
    // create a ref to store the textInput DOM element
    this.textInput = React.createRef();
    this.focusTextInput = this.focusTextInput.bind(this);
  }

  focusTextInput() {
    // Explicitly focus the text input using the raw DOM api
    // Note: we're accessing "current" to get the DOM node
    this.textInput.current.focus();
  }

  render() {
    // tell React that we want to associate the <input> ref
    // with the `textInput` that we created in the constructor
    return (
      <div>
        <input
          type="text"
          ref={this.textInput} />

        <input
          type="button"
          value="Focus the text input"
          onClick={this.focusTextInput}
        />
      </div>
    );
  }
}