FadeIn on Scroll (jQuery)

Used to answer a question on StackOverflow.

by Phil Button

HTML

<div>
    <div class="fade">Fade In 01</div>
    <div class="fade">Fade In 02</div>
    <div class="fade">Fade In 03</div>
    <div class="fade">Fade In 04</div>
    <div class="fade">Fade In 05</div>
    <div class="fade">Fade In 06</div>
    <div class="fade">Fade In 07</div>
    <div class="fade">Fade In 08</div>
    <div class="fade">Fade In 09</div>
    <div class="fade">Fade In 10</div>
</div>

CSS

.fade {
    margin: 50px;
    padding: 100px;
    padding-top: 0;
    background-color: lightgreen;
    min-height: 25vh;
}

JavaScript

$(window).on("load",function() {
    function fade() {
        var animation_height = $(window).innerHeight() * 0.25;
        var ratio = Math.round( (1 / animation_height) * 10000 ) / 10000;

        $('.fade').each(function() {
            
            var objectTop = $(this).offset().top;
            var windowBottom = $(window).scrollTop() + $(window).innerHeight();
            var windowTop = $(window).scrollTop();
            
            if ( objectTop < windowBottom ) {
                if ( objectTop < windowBottom - animation_height ) {
                    $(this).html( 'fully visible' );
                    $(this).css( {
                        transition: 'opacity 0.1s linear',
                        opacity: 1
                    } );

                } else {
                    $(this).html( 'fading in/out' );
                    $(this).css( {
                        transition: 'opacity 0.25s linear',
                        opacity: (windowBottom - objectTop) * ratio
                    } );
                }
            } else if (objectTop < windowTop) {
                if ( objectTop < windowTop + animation_height ) {
                    $(this).html( 'fully visible' );
                    $(this).css( {
                        transition: 'opacity 0.1s linear',
                        opacity: 1
                    } );

                } else {
                    $(this).html( 'fading in/out' );
                    $(this).css( {
                        transition: 'opacity 0.25s linear',
                        opacity: (windowBottom - objectTop) * ratio
                    } );
                }
            } else {
                $(this).html( 'not visible' );
                $(this).css( 'opacity', 0 );
            }
        });
    }
    $('.fade').css( 'opacity', 0 );
    fade();
    $(window).scroll(function() {fade();});
});