如何修复：每次渲染后，React Hook useEffect 中对“主题”变量的赋值将丢失

Question

I am a newbie in React, and I am working on a dark-mode function. It has a warning and I would like to ask what is the best way to fix it. Thanks

  20:21  warning  Assignments to the 'theme' variable from inside React Hook useEffect will be lost after each render. To preserve the value over time, store
it in a useRef Hook and keep the mutable value in the '.current' property. Otherwise, you can move this variable directly inside useEffect

import React from "react";
import { RiContrastLine } from 'react-icons/ri';
import { useEffect } from "react";


const DarkMode = () => {

    let body = "";
    if (typeof document !== "undefined") {
        body = document.body;
    }

    let clickedClass = "clicked";
    const lightTheme = "light-mode";
    const darkTheme = "dark-mode";
    let theme;

    useEffect(() => {
        if (localStorage) {
            theme = localStorage.getItem("theme");
            if (theme === darkTheme) {
            }
        }

        if (theme === lightTheme || theme === darkTheme) {
            body.classList.add(theme);
        } else {
            body.classList.add(lightTheme);
        }
    }, [])

    const switchTheme = (e) => {
        if (theme === darkTheme) {
            body.classList.replace(darkTheme, lightTheme);
            localStorage.setItem("theme", "light-mode");
            e.target.classList.remove(clickedClass);
            theme = lightTheme;

        } else {
            body.classList.replace(lightTheme, darkTheme);
            localStorage.setItem("theme", "dark-mode");
            e.target.classList.add(clickedClass);
            theme = darkTheme;
        }
    };



    return (
        <button className='general-menu-button' type="button" onClick={(e) => switchTheme(e)}>
            <RiContrastLine />
        </button>
    );
};

export default DarkMode;

I have tried to add useRef, but with my limited understanding, it turned out another error.

I would like to know how to resolve the warning

Answer 1

Why not store the theme in state? This will persist the value across rerenders

Import useState at the top of your file:

import { useState } from 'react'

replace

let theme;

with

const [theme, setTheme] = useState(/* initial theme here */)

then replace anytime you assign theme to a setTheme call eg

theme = localStorage.getItem('theme')
// becomes
setTheme(localStorage.getItem('theme'))