构建深色模式切换器
实现一个 JavaScript 切换器，在 html 元素上添加和移除 dark class，并将偏好保存到 localStorage。
构建深色模式切换器 是 CoddyKit 上的免费 Tailwind CSS Academy 课时。 这是第 3 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 Tailwind CSS Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Tailwind CSS Academy 课程共包含 4 节课。
本课时的部分内容尚未翻译，以英文显示。
What the Toggle Needs to Do
A dark mode toggle needs to accomplish three things: read the user's saved preference (or OS preference as a fallback), apply the correct class to the <html> element on page load, and update the class and save the preference when the user clicks the toggle button. Getting the initial load right is critical — applying the class before the page renders prevents a flash of incorrect theme.
Preventing Flash of Wrong Theme
The most common dark mode bug is a flash of light theme when a user in dark mode refreshes the page. This happens because JavaScript runs after the HTML has already been parsed and rendered. The fix is to run a tiny inline <script> in the <head> — before any CSS or body content — that reads localStorage and immediately adds the dark class if needed.
<!-- Place this FIRST inside <head> -->
<script>
if (localStorage.theme === 'dark' ||
(!('theme' in localStorage) && window.matchMedia('(prefers-color-scheme: dark)').matches)) {
document.documentElement.classList.add('dark');
}
</script>HTML Structure for the Toggle
The toggle button itself can be as simple as a <button> with sun and moon icons. Show the sun icon in dark mode (to switch to light) and the moon icon in light mode (to switch to dark). Use Tailwind's hidden and block with dark: variants to swap which icon is visible based on the current mode.
<button id="theme-toggle" class="p-2 rounded-lg bg-gray-200 dark:bg-gray-700">
<!-- Moon icon (visible in light mode) -->
<svg id="icon-moon" class="w-5 h-5 text-gray-800 dark:hidden" fill="currentColor" viewBox="0 0 20 20">
<path d="M17.293 13.293A8 8 0 016.707 2.707a8.001 8.001 0 1010.586 10.586z" />
</svg>
<!-- Sun icon (visible in dark mode) -->
<svg id="icon-sun" class="w-5 h-5 text-yellow-400 hidden dark:block" fill="currentColor" viewBox="0 0 20 20">
<path d="M10 2a1 1 0 100 2 1 1 0 000-2zm0 14a1 1 0 100 2 1 1 0 000-2z" />
</svg>
</button>Writing the Toggle JavaScript
The toggle function checks whether the dark class is currently present on <html>. If it is, the function removes it and saves 'light' to localStorage. If it is not, the function adds it and saves 'dark'. This pattern is clean, reversible, and works without any external libraries or frameworks.
const toggle = document.getElementById('theme-toggle');
toggle.addEventListener('click', () => {
const html = document.documentElement;
if (html.classList.contains('dark')) {
html.classList.remove('dark');
localStorage.setItem('theme', 'light');
} else {
html.classList.add('dark');
localStorage.setItem('theme', 'dark');
}
});Respecting OS Preference as Default
If the user has never interacted with your toggle, there is nothing in localStorage. In that case, you should fall back to the OS prefers-color-scheme media query. The window.matchMedia API lets you check this at runtime. If the OS is in dark mode and the user has not overridden it, apply the dark class so the initial experience matches their system setting.
function getInitialTheme() {
const saved = localStorage.getItem('theme');
if (saved) return saved;
// No saved preference — use OS setting
return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
}
if (getInitialTheme() === 'dark') {
document.documentElement.classList.add('dark');
}Full Toggle Implementation
Putting it all together: the inline head script handles initial load without flash, the button HTML uses dark: variants to swap icons, and the click handler toggles the class and saves to localStorage. This complete pattern is the standard approach used by most Tailwind-based apps. It requires no dependencies and works in any environment.
<!-- In <head> (anti-flash) -->
<script>
const saved = localStorage.getItem('theme');
const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
if (saved === 'dark' || (!saved && prefersDark)) {
document.documentElement.classList.add('dark');
}
</script>
<!-- In body JS -->
<script>
document.getElementById('theme-toggle').addEventListener('click', () => {
const isDark = document.documentElement.classList.toggle('dark');
localStorage.setItem('theme', isDark ? 'dark' : 'light');
});
</script>Toggle Button With Smooth Transition
Add a satisfying animation to the theme switch by putting a transition-colors class on your body or root container. This makes all color changes animate smoothly instead of snapping instantly. Keep the transition duration short — around 200-300ms — to feel responsive. Apply it at the highest container level so all child elements inherit the smooth transition behavior.
<body class="bg-white dark:bg-gray-900 text-gray-900 dark:text-gray-100 transition-colors duration-200">
<!-- All children now transition smoothly between light and dark -->
</body>Toggle in a React Component
In React, manage the dark mode state with useState and synchronize it with the DOM and localStorage using useEffect. A common pattern is a custom useDarkMode hook that encapsulates all the logic. Trigger the DOM class change inside the effect so it stays in sync with the React state.
import { useState, useEffect } from 'react';
function useDarkMode() {
const [dark, setDark] = useState(() => localStorage.getItem('theme') === 'dark');
useEffect(() => {
document.documentElement.classList.toggle('dark', dark);
localStorage.setItem('theme', dark ? 'dark' : 'light');
}, [dark]);
return [dark, setDark];
}Toggle in a Next.js App
In Next.js, the anti-flash script cannot be easily placed in <head> via a component because React hydration runs after the HTML is parsed. The recommended approach is to add the inline script via next/script with strategy='beforeInteractive', or to place it directly in a custom _document.js file inside the Head component. This ensures the script runs synchronously before the page renders.
// pages/_document.js (Next.js Pages Router)
import { Html, Head, Main, NextScript } from 'next/document';
export default function Document() {
return (
<Html>
<Head>
<script dangerouslySetInnerHTML={{ __html: `
const t = localStorage.getItem('theme');
const d = window.matchMedia('(prefers-color-scheme: dark)').matches;
if (t === 'dark' || (!t && d)) document.documentElement.classList.add('dark');
` }} />
</Head>
<body><Main /><NextScript /></body>
</Html>
);
}Listening to OS Changes at Runtime
Users sometimes change their OS theme while your app is open. You can listen for these changes using window.matchMedia's change event. Only apply the OS preference change if the user has not set an explicit preference in your app — otherwise you would override their in-app choice. This provides the most respectful user experience.
const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
mediaQuery.addEventListener('change', (e) => {
// Only auto-update if the user hasn't set an explicit preference
if (!localStorage.getItem('theme')) {
document.documentElement.classList.toggle('dark', e.matches);
}
});Toggle Accessibility Considerations
The dark mode toggle button should be keyboard accessible and have a meaningful aria-label that updates to reflect the current action. Use aria-pressed to communicate the current state to screen readers. Since only a visual icon changes, screen reader users need the label to understand what pressing the button will do.
<button
id="theme-toggle"
aria-label="Switch to dark mode"
aria-pressed="false"
class="p-2 rounded-lg bg-gray-200 dark:bg-gray-700"
>
<!-- icons -->
</button>
<script>
const btn = document.getElementById('theme-toggle');
btn.addEventListener('click', () => {
const isDark = document.documentElement.classList.toggle('dark');
btn.setAttribute('aria-pressed', isDark);
btn.setAttribute('aria-label', isDark ? 'Switch to light mode' : 'Switch to dark mode');
localStorage.setItem('theme', isDark ? 'dark' : 'light');
});
</script>Quick Check
Test your understanding of building a dark mode toggle with Tailwind CSS.
Lesson Recap
In this lesson you learned: an inline head script prevents the flash of wrong theme on page load, classList.toggle('dark') combined with localStorage creates a persistent user toggle, and matchMedia lets you respect the OS preference as a default when no user preference is saved. Next up we apply dark mode to complex components like cards, navbars, and modals.
常见问题解答
「构建深色模式切换器」课时是免费的吗？
是的 — 「构建深色模式切换器」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Tailwind CSS Academy 课程的其余内容，请升级到 CoddyKit PRO。 Tailwind CSS Academy 课程共包含 4 节课。
「构建深色模式切换器」这节课中我会学到什么？
实现一个 JavaScript 切换器，在 html 元素上添加和移除 dark class，并将偏好保存到 localStorage。 你通过在浏览器中直接运行的动手代码来练习 Tailwind CSS Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。
学习 Tailwind CSS Academy 需要有经验吗？
无需任何先前经验。CoddyKit 上的 Tailwind CSS Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 3 节课，共 4 节。
「构建深色模式切换器」课时需要多长时间？
大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。
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能。每节 Tailwind CSS Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。