Dark Mode Toggle in HTML Web Components

When designing my digital garden, I knew I wanted a cute dark mode toggle. Once I had drawn my SVG, I started building a web component that had all the same functionality as my dark mode toggle in React. This includes everything I learned while accessibility auditing my site.

In addition to changing the theme, the toggle needs to take the user's prefers-color-scheme selection into account and persist the user's preference across reloads. For accessibility, the toggle's screen reader announcement must make sense (e.g. "dark mode toggle on"). Since I want to display an SVG instead of a checkbox with text, I'll have to add focus and hover styling and a label that shows up on hover.

Toggle Web Component

First, I need a Toggle class that creates an HTML element. Using the Custom Element API, I'll define <toggle-component> using this class.

Using the class's constructor, I set the innerHTML of <toggle-component> to a <label> with an <input type="checkbox"> and SVG as children. The label has a title attribute, giving our toggle a visible "dark mode toggle" label if a user hovers over it or the <input>. Because the checkbox is a child of the <label>, interacting with the <label> is the same thing as interacting with the <input> and vice versa. The title attribute also makes sure screen readers announce "dark mode toggle" when a user interacts with this component.

Once the HTML is in place, I add a connectedCallback function to the class. This part of the custom elements API defines functions for use within the component and executes code when the component is inserted into the DOM.

// /components/toggle.js

class Toggle extends HTMLElement {
  constructor() {
    super();
    this.innerHTML = `
      <label title="dark mode toggle">
        <input type="checkbox" id="theme-toggle" class="theme-switch" />
        <svg id="daisy">{SVG code removed for brevity}</svg>
      </label>
    `
    this.setAttribute("class", "toggle-component");
  }

  connectedCallback() {
    function switchTheme(e) {
      if (e.target.checked) {
        setTheme('dark');
        return;
      }

      setTheme('light');   
    };

    function setTheme(themeName) {
      localStorage.setItem('theme', themeName);
      document.documentElement.setAttribute('data-theme', themeName);
    };

    function setCheckBox(toggleSwitch, theme) {
      toggleSwitch.checked = theme === 'dark' ? true : false;
    }

    function keepTheme() {
      const toggleSwitch = document.querySelector('#theme-toggle');
      toggleSwitch.addEventListener('change', switchTheme, false);
      const theme = localStorage.getItem('theme');
      if (theme) {
        setTheme(theme);
        setCheckBox(toggleSwitch, theme);
        return;
      };

      const prefersLightTheme = window.matchMedia('(prefers-color-scheme: light)');
      if (prefersLightTheme.matches) {
        setTheme('light');
        return;
      };

      setTheme('dark');
      setCheckBox(toggleSwitch, 'dark');
    };

    document.addEventListener("DOMContentLoaded", keepTheme);    
  }
}

customElements.define("toggle-component", Toggle);

Because <toggle-component> is inserted into the DOM before the page loads, the only code that executes immediately adds an event listener. The event listener calls keepTheme as soon as the page has loaded. First, keepTheme adds an event listener to the <input> that calls switchTheme when a user interacts with it. switchTheme passes 'dark' to setTheme if the checkbox is checked and 'light' if it is not. The string passed to setTheme is set as the CSS theme and saved in localStorage which will persist across reloads.

The rest of keepTheme is dedicated to choosing the right theme on load. First, it checks localStorage to see if the user's preference is already set. Next, it checks if prefers-color-scheme is set to 'light'. Finally, it defaults to dark mode. For both dark and light mode, I call setTheme. For dark mode, I also call setCheckbox. The checkbox mounts in an unchecked state. A screen reader will announce "dark mode" and whether the checkbox is checked. To get an announcement like "dark mode toggle checked" or "dark mode toggle on", I have to programmatically check the checkbox when I set the theme to 'dark' on load.

Toggle Styling

I chose to draw a fairly simple design so I could put my SVG code directly in the web component and programmatically change the fill color. This way, the background color of the daisy always matches the theme. Next, I use opacity: 0; to hide the checkbox and position it in the middle of the image. Finally, I add the hover and focus styles.

/* /styles/styles.css */

[data-theme="light"] {
  --toggle-background: #242D54;
}

[data-theme="dark"] {
  --toggle-background: #282e53;
}

#daisy path {
  fill: var(--toggle-background);
}

.theme-switch {
  position: relative;
  bottom: 30px;
  left: 55px;
  width: 1em;
  height: 1em;
  opacity: 0;
}

.theme-switch:focus + #daisy path,
.theme-switch:hover + #daisy path {
  fill: white;
}

.theme-switch:focus + #daisy {
  outline: 3px solid white;
  outline-offset: 5px;
}

Using the Toggle Web Component

All I need to do is import my stylesheet and component script in the <head> of an HTML page. Then I can call <toggle-component> anywhere in the page.

<!-- index.html -->

<html lang="es">
  <head>
    <link rel='stylesheet' href='../styles/styles.css'/>
    <script src="../components/toggle.js" type="text/javascript" defer></script>
  </head>
  <body>
    <header>
      <toggle-component></toggle-component>
    </header>
  </body>
</html>

Conclusion

I had fun getting my dark mode toggle to work just as well in a web component as it does in React. You can see this live in my digital garden and the full code in the GitHub repo.