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Back on dry land. Take your blog from day to night, no island required!

Now that you can build Astro islands for interactive elements, don’t forget that you can go pretty far with just vanilla JavaScript and CSS!

Let’s build a clickable icon to let your users toggle between light or dark mode using another <script> tag for interactivity… with no framework JavaScript sent to the browser.

Get ready to…

  • Build an interactive theme toggle with only JavaScript and CSS
  • Send as little JavaScript to the browser as possible!

Add and style a theme toggle icon

Section titled Add and style a theme toggle icon

  1. Create a new file at src/components/ThemeIcon.astro and paste the following code into it:

    src/components/ThemeIcon.astro
    ---
    ---
    <button id="themeToggle">
      <svg width="30px" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24">
        <path class="sun" fill-rule="evenodd" d="M12 17.5a5.5 5.5 0 1 0 0-11 5.5 5.5 0 0 0 0 11zm0 1.5a7 7 0 1 0 0-14 7 7 0 0 0 0 14zm12-7a.8.8 0 0 1-.8.8h-2.4a.8.8 0 0 1 0-1.6h2.4a.8.8 0 0 1 .8.8zM4 12a.8.8 0 0 1-.8.8H.8a.8.8 0 0 1 0-1.6h2.5a.8.8 0 0 1 .8.8zm16.5-8.5a.8.8 0 0 1 0 1l-1.8 1.8a.8.8 0 0 1-1-1l1.7-1.8a.8.8 0 0 1 1 0zM6.3 17.7a.8.8 0 0 1 0 1l-1.7 1.8a.8.8 0 1 1-1-1l1.7-1.8a.8.8 0 0 1 1 0zM12 0a.8.8 0 0 1 .8.8v2.5a.8.8 0 0 1-1.6 0V.8A.8.8 0 0 1 12 0zm0 20a.8.8 0 0 1 .8.8v2.4a.8.8 0 0 1-1.6 0v-2.4a.8.8 0 0 1 .8-.8zM3.5 3.5a.8.8 0 0 1 1 0l1.8 1.8a.8.8 0 1 1-1 1L3.5 4.6a.8.8 0 0 1 0-1zm14.2 14.2a.8.8 0 0 1 1 0l1.8 1.7a.8.8 0 0 1-1 1l-1.8-1.7a.8.8 0 0 1 0-1z"/>
        <path class="moon" fill-rule="evenodd" d="M16.5 6A10.5 10.5 0 0 1 4.7 16.4 8.5 8.5 0 1 0 16.4 4.7l.1 1.3zm-1.7-2a9 9 0 0 1 .2 2 9 9 0 0 1-11 8.8 9.4 9.4 0 0 1-.8-.3c-.4 0-.8.3-.7.7a10 10 0 0 0 .3.8 10 10 0 0 0 9.2 6 10 10 0 0 0 4-19.2 9.7 9.7 0 0 0-.9-.3c-.3-.1-.7.3-.6.7a9 9 0 0 1 .3.8z"/>
      </svg>
    </button>
    

    <style>
      #themeToggle {
        border: 0;
        background: none;
      }
      .sun { fill: black; }
      .moon { fill: transparent; }
    

      :global(.dark) .sun { fill: transparent; }
      :global(.dark) .moon { fill: white; }
    </style>

  2. Add the icon to Header.astro so that it will be displayed on all pages. Don’t forget to import the component.

    src/components/Header.astro
    ---
    import Hamburger from './Hamburger.astro';
    import Navigation from './Navigation.astro';
    import ThemeIcon from './ThemeIcon.astro';
    ---
    <header>
      <nav>
        <Hamburger />
        <ThemeIcon />
        <Navigation />
      </nav>
    </header>

  3. Visit your browser preview at http://localhost:4321 to see the icon now on all your pages. You can try clicking it, but you have not written a script to make it interactive yet.

Add CSS styling for a dark theme

Section titled Add CSS styling for a dark theme

Choose some alternate colors to use in dark mode.

  1. In global.css, define some dark styles. You can choose your own, or copy and paste:

    src/styles/global.css
    html.dark {
      background-color: #0d0950;
      color: #fff;
    }
    

    .dark .nav-links a {
      color: #fff;
    }

Add client-side interactivity

Section titled Add client-side interactivity

To add interactivity to an Astro component, you can use a <script> tag. This script can check and set the current theme from localStorage and toggle the theme when the icon is clicked.

  1. Add the following <script> tag in src/components/ThemeIcon.astro after your <style> tag:

    src/components/ThemeIcon.astro
    <style>
      .sun { fill: black; }
      .moon { fill: transparent; }
    

      :global(.dark) .sun { fill: transparent; }
      :global(.dark) .moon { fill: white; }
    </style>
    

    <script is:inline>
      const theme = (() => {
        if (typeof localStorage !== 'undefined' && localStorage.getItem('theme')) {
          return localStorage.getItem('theme');
        }
        if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
          return 'dark';
        }
          return 'light';
      })();
    

      if (theme === 'light') {
        document.documentElement.classList.remove('dark');
      } else {
        document.documentElement.classList.add('dark');
      }
    

      window.localStorage.setItem('theme', theme);
    

      const handleToggleClick = () => {
        const element = document.documentElement;
        element.classList.toggle("dark");
    

        const isDark = element.classList.contains("dark");
        localStorage.setItem("theme", isDark ? "dark" : "light");
      }
    

      document.getElementById("themeToggle").addEventListener("click", handleToggleClick);
    </script>

  2. Check your browser preview at http://localhost:4321 and click the theme icon. Verify that you can change between light and dark modes.

Test your knowledge

Section titled Test your knowledge

Choose whether each of the following statements describes Astro <script> tags, UI framework components, or both.

  1. They allow you to include interactive UI elements on your website.

  2. They will create static elements on your site unless you include a client: to send their JavaScript to the client and run in the browser.

  3. They allow you to “try out” a new framework without requiring you to start an entirely new project using that tech stack.

  4. They allow you to reuse code you have written in other frameworks and you can often just drop them right into your site.

  5. They allow you to add interactivity without needing to know or learn any other JavaScript frameworks.

Checklist

Section titled Checklist

Resources

Section titled Resources