Components not reacting to Alpine.JS $store variable

Question

I've got 2 Alpine components on one page that both should react on one single global store variable. Using the console I've verified that my global store variable does update, however my components still do not react to changes of $store.scrolled

Why are my <header> and <main> components not effected by $store.scrolled ?

<script src="https://unpkg.com/@alpinejs/intersect@3.x.x/dist/cdn.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/alpinejs/3.10.5/cdn.js"></script>
<script src="https://cdn.tailwindcss.com"></script>
<div class="pt-10 pl-[100px]">
  <data id="global_helper" 
        class="absolute top-0 left-0 h-[300px] w-[100px] bg-yellow-500 z-[-1]" 
        x-data x-init="$store.scrolled=false" 
        x-intersect:enter="$store.scrolled=false" 
        x-intersect:leave="$store.scrolled=true">300px</data>
  <header x-data 
          class="fixed block w-full h-10 top-0" 
          :class="$store.scrolled ? 'bg-red-500' : 'bg-blue-500'">
    This is the Header, the site is <span x-text="$store.scrolled ? 'in scrolldown mode' : 'on the top'"></span>
  </header>
  <main x-data 
        class="p-4" 
        :class="$store.scrolled ? 'bg-gray-100' : 'bg-green-100'">
    <h1 class="text-2xl pb-10 font-bold">The scroll position should effect both, the header and the main content.</h1>

    <p class="pb-10">
      After scrolling down 300px both header and main should change their background color.<br>
      The header and the main content are intentionally NOT in the same <tt>x-data</tt> component.<br>
      (The parent of those would be the body or a similar large node)
    </p>
    <p>
    Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.
    Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi. Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat.
    Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat. Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi.
    Nam liber tempor cum soluta nobis eleifend option congue nihil imperdiet doming id quod mazim placerat facer possim assum. Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat.
    </p>
  </main>

</div>

Answer 1

You have to register the store object before running Alpine.js, you cannot create reactive store attributes on the fly. Here is the fixed example, please uncomment the document.addEventListener('alpine:init', () => {}) lines in the real code, SO code snippet does not fires the alpine:init event, so I disabled it here to make the example work.

<script src="https://unpkg.com/@alpinejs/intersect@3.x.x/dist/cdn.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/alpinejs/3.10.5/cdn.js"></script>
<script src="https://cdn.tailwindcss.com"></script>
<script>
//document.addEventListener('alpine:init', () => {
    Alpine.store('scrolled', false)
//})
</script>
<div class="pt-10 pl-[100px]">
  <data id="global_helper" 
        class="absolute top-0 left-0 h-[300px] w-[100px] bg-yellow-500 z-[-1]" 
        x-data x-init="$store.scrolled=false" 
        x-intersect:enter="$store.scrolled=false" 
        x-intersect:leave="$store.scrolled=true">300px</data>
  <header x-data 
          class="fixed block w-full h-10 top-0" 
          :class="$store.scrolled ? 'bg-red-500' : 'bg-blue-500'">
    This is the Header, the site is <span x-text="$store.scrolled ? 'in scrolldown mode' : 'on the top'"></span>
  </header>
  <main x-data x-init="console.log('loaded')"
        class="p-4" 
        :class="$store.scrolled ? 'bg-gray-100' : 'bg-green-100'">
    <h1 class="text-2xl pb-10 font-bold">The scroll position should effect both, the header and the main content.</h1>

    <p class="pb-10">
      After scrolling down 300px both header and main should change their background color.<br>
      The header and the main content are intentionally NOT in the same <tt>x-data</tt> component.<br>
      (The parent of those would be the body or a similar large node)
    </p>
    <p>
    Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.
    Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi. Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat.
    Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat. Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi.
    Nam liber tempor cum soluta nobis eleifend option congue nihil imperdiet doming id quod mazim placerat facer possim assum. Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat.
    </p>
  </main>

</div>