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title: Dark Mode Toggle
description: Dark mode without the flash of default theme
date: 2021-04-21
draft: false
slug: /pensieve/dark-mode-toggle
tags:
- Theming
- Dark Mode
---
Dark mode toggle without the flash of default theme. Important bits:
- CSS variables for color theming
- Put `data-theme` attribute on `<html>`, not `<body>`, so we can run the JS before the DOM finishes rendering
- Run local storage check in the `<head>`
- JS for toggle button click handler can come after render
## HTML
```html
<!DOCTYPE html>
<html lang="en" data-theme="light">
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
...
<script>
// If there's a theme stored in localStorage, use it on the <html>
const localStorageTheme = localStorage.getItem('theme');
if (localStorageTheme) {
document.documentElement.setAttribute('data-theme', localStorageTheme);
}
</script>
</head>
<body>
<div class="theme-toggle">
<button
class="theme-toggle-btn js-theme-toggle"
aria-label="Activate dark mode"
title="Activate dark mode"
>
<!--
<svg class="light-mode">
<use xlink:href="#sun"></use>
</svg>
<svg class="dark-mode">
<use xlink:href="#moon"></use>
</svg>
-->
</button>
</div>
<script src="app.js"></script>
</body>
</html>
```
## CSS Variables
```css
:root {
--bg: #ffffff;
--text: #000000;
}
[data-theme='dark'] {
--bg: #000000;
--text: #ffffff;
}
```
## JavaScript
```js:title=app.js
const themeToggleBtn = document.querySelector('.js-theme-toggle');
themeToggleBtn.addEventListener('click', () => onToggleClick());
const onToggleClick = () => {
const { theme } = document.documentElement.dataset;
const themeTo = theme && theme === 'light' ? 'dark' : 'light';
const label = `Activate ${theme} mode`;
document.documentElement.setAttribute('data-theme', themeTo);
localStorage.setItem('theme', themeTo);
themeToggleBtn.setAttribute('aria-label', label);
themeToggleBtn.setAttribute('title', label);
};
```
## Resources
- <https://css-tricks.com/a-complete-guide-to-dark-mode-on-the-web/>
- <https://css-tricks.com/flash-of-inaccurate-color-theme-fart/>
- <https://mxb.dev/blog/color-theme-switcher/>
- <https://www.joshwcomeau.com/react/dark-mode/>
- <https://web.dev/prefers-color-scheme/>
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