How can I detect element visibility during scrolling with jQuery?

Detecting Element Visibility During Scrolling with jQuery

When navigating a web page with extensive content, it becomes essential to trigger specific actions when users reach particular elements. This article demonstrates how to use jQuery to trigger an event when a user scrolls to a designated HTML element.

Setting Up the Element

Consider an HTML page with an h1 element positioned far down:

<code class="html"><h1 id="scroll-to">TRIGGER EVENT WHEN SCROLLED TO.</h1></code>

Triggering the Event with jQuery

The objective is to display an alert when the user's browser view reaches the h1 element. While one may initially attempt to use the scroll() method on the h1 element:

<code class="js">$('#scroll-to').scroll(function() {
    alert('you have scrolled to the h1!');
});</code>

this approach will not yield the desired result. Instead, it is necessary to calculate the element's offset relative to the page and compare it to the scroll position.

The Solution

<code class="js">$(window).scroll(function() {
    var hT = $('#scroll-to').offset().top,
        hH = $('#scroll-to').outerHeight(),
        wH = $(window).height(),
        wS = $(this).scrollTop();
    if (wS > (hT + hH - wH)) {
        console.log('H1 on the view!');
    }
});</code>

The above code checks if the bottom of the h1 element has been reached within the visible portion of the page.

Enhancements

• Demo Fiddle: Experience the functionality firsthand.
• FadeIn() instead of Alert: Use fadeIn() to seamlessly animate the element's visibility.
• Element Visibility Check: Modify the script to detect if the element is completely within the viewport, regardless of scrolling direction.

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