web/_includes/mode-toggle.html

<!--
  Switch the mode between dark and light.

  v2.1
  https://github.com/cotes2020/jekyll-theme-chirpy
  © 2020 Cotes Chung
  MIT License
-->

<i class="mode-toggle fas fa-sun" dark-mode-invisible></i>
<i class="mode-toggle fas fa-moon" light-mode-invisible></i>

<script type="text/javascript">

  class ModeToggle {
    static get MODE_KEY() { return "mode"; }
    static get DARK_MODE() { return "dark"; }
    static get LIGHT_MODE() { return "light"; }

    constructor() {
      if (this.mode != null) {
        if (this.mode == ModeToggle.DARK_MODE) {
          if (!this.isSysDarkPrefer) {
            this.setDark();
          }
        } else {
          if (this.isSysDarkPrefer) {
            this.setLight();
          }
        }
      }

      var self = this;

      /* always follow the system prefers */
      this.sysDarkPrefers.addListener(function() {
        if (self.mode != null) {
          if (self.mode == ModeToggle.DARK_MODE) {
            if (!self.isSysDarkPrefer) {
              self.setDark();
            }

          } else {
            if (self.isSysDarkPrefer) {
              self.setLight();
            }
          }

          self.clearMode();
        }
      });

    } /* constructor() */


    setDark() {
      $('html').attr(ModeToggle.MODE_KEY, ModeToggle.DARK_MODE);
      sessionStorage.setItem(ModeToggle.MODE_KEY, ModeToggle.DARK_MODE);
    }

    setLight() {
      $('html').attr(ModeToggle.MODE_KEY, ModeToggle.LIGHT_MODE);
      sessionStorage.setItem(ModeToggle.MODE_KEY, ModeToggle.LIGHT_MODE);
    }

    clearMode() {
      $('html').removeAttr(ModeToggle.MODE_KEY);
      sessionStorage.removeItem(ModeToggle.MODE_KEY);
    }

    get sysDarkPrefers() { return window.matchMedia("(prefers-color-scheme: dark)"); }

    get isSysDarkPrefer() { return this.sysDarkPrefers.matches; }

    get isDarkMode() { return this.mode == ModeToggle.DARK_MODE; }

    get isLightMode() { return this.mode == ModeToggle.LIGHT_MODE; }

    get hasMode() { return this.mode != null; }

    get mode() { return sessionStorage.getItem(ModeToggle.MODE_KEY); }

    flipMode() {
      if (this.hasMode) {
        if (this.isSysDarkPrefer) {
          if (this.isLightMode) {
            this.clearMode();
          } else {
            this.setLight();
          }

        } else {
          if (this.isDarkMode) {
            this.clearMode();
          } else {
            this.setDark();
          }
        }

      } else {
        if (this.isSysDarkPrefer) {
          this.setLight();
        } else {
          this.setDark();
        }
      }

    } /* flipMode() */

  } /* ModeToggle */

  let toggle = new ModeToggle();

  $(".mode-toggle").click(function() {
    toggle.flipMode();
  });

</script>