尝试访问脚本时使用 useEffect 在我的 React 项目中获取“TypeError：无法读取 null 的属性 'classList'”？

Question

I am trying to implement useEffect on my React project to add my external JS file in.

This is my React code currently, with the commented out list being what I am trying to access from my script.

import React, { useEffect } from 'react';
import { Link } from 'react-router-dom';
import './HeroSec.css';
import '../script.js';

function HeroSec(){

    useEffect(() => {
        console.log('render');

        const script = document.createElement('script');

        script.src = "../script.js";
        script.async = true;
      
        document.body.appendChild(script);
      
        return () => {
          document.body.removeChild(script);
        }
    })

    return(
    <div className='hero-container'>

        <h1>Example Name </h1> 
        {/*
        <ul id="foo" class="hidden-text">
        <li> E </li>
        <li class="hidden">x</li>
        <li class="hidden">a</li>
        <li class="hidden">m</li>
        <li class="hidden">p</li>
        <li class="hidden">l</li>
        <li class="hidden">e</li>
        <li class="spaced">N</li>
        <li class="hidden">a</li>
        <li class="hidden">m</li>
        <li class="hidden">e</li>
        </ul>
        */}

and this is the script.js file I am getting the error in.

const text = document.querySelector("#foo");
    setTimeout(() => {
        text.classList.toggle("hidden-text");
    }, 1000);

Essentially, I am just trying to grab that list element in React, and use the script to set a timeout so I can toggle off the hidden-text class. Of course there is some CSS that does this separately.

Hopefully I made it easy to understand the bigger picture of what I am trying to do and any help on that would be appreciated. In terms of the specific TypeError I am getting, is there maybe another property instead of classList that I can use to access the list from within React?

Answer 1

This is not the "react" way of doing things, you should not be getting direct references to html elements and modifying them in React. I did not quite understand what you want to do but you can try below approach to render components differently according to the state.

function HeroSec(){
  const [toggled,setToggled] = useState(false);
  ...

 return ( ....
 //Component will have different class based on the value of toggled and
 // when toggled value changed react will re-render the element with new value
 <ul id="foo" class={toggled ? : "hidden-text" : <another_classname>} >
  ...)

Then use setToggled in your setTimeout function to change toggled from false to true.

Answer 2

Ideally you use use the useRef hook to access element with React function components. For example:

  export default function HeroSec() {
  const listElement = useRef(null);

  useEffect(() => {
    setTimeout(() => {
      listElement.current.classList.add("hidden-text");
    }, 1000);
    return () => {};
  });

  return (
    <div className="hero-container">
      <h1>Example Name </h1>
      <ul ref={listElement} id="foo">
        <li> E </li>
        <li className="hidden">x</li>
        <li className="hidden">a</li>
      </ul>
    </div>
  );
}

Sandbox: https://codesandbox.io/s/frosty-cache-6rkdq?file=/src/App.js