[SOLVED] Back to top button in Alpine.js

I've been practising with Alpine.JS and attempted to create a back to top button. I've got stuck as the button does not hide at all, I'm trying to get Alpine to recognise when the user scrolls down to a certain height, then show the button.

"is-hidden" is a bulma css property

Here is the code:

<body x-data="{show_backToTop: true}" 
    @scroll.window="show_backToTop = (window.pageYOffset > 200) ? false : true">

<!-- back to top scroll button -->
<div class="show_backToTop ? 'block' : 'is-hidden' ">
    <div id="back_To_Top" 
        x-ref="backTotop" 
        x-data @click="window.scrollTo({top: 0, behavior: 'smooth'})" // ignore this, this is the script to return to the top of page
        class="scrollTop">
        <p>Back to top</p>
    </div>
</div>
</body>

Solution

One problem is in the class of the button: to make it dynamic you have to precede it with a colon - more info here
Another problem is in the logic: the show_backToTop flag should be initialized to false and set to true if the page is scrolled down

Here a working example - I have reduced the offset scroll to 30

<body x-data="{show_backToTop: false}"
      @scroll.window="show_backToTop = window.pageYOffset > 30"
>
   <!-- simple style for the sticky button --> 
   <style> 
       .scrollTop{
           position: fixed;
           top: 0;
           overflow: hidden;
           z-index: 10;
       }
   </style>

    <div :class="show_backToTop ? 'block' : 'is-hidden'">

        <div id="back_To_Top"
             x-ref="backTotop"
             @click="window.scrollTo({top: 0, behavior: 'smooth'})"
             class="scrollTop"
        >

        <p>Back to top</p>
        </div>

    </div>

    <table style="width:100%">   <!-- this is only to fill out the page for testing -->

        <template x-for="i in 100">

            <tr>
                <td x-text="i"></td>
                <td>aaa</td>
                <td>bbb</td>
                <td>ccc</td>
                <td>ddd</td>
            </tr>

        </template>

    </table>

</body>