How much of an element is visible in viewport

Question

There's a div (brown rectangle) on the page. The page is higher than the viewport (orange rectangle) so it can be scrolled, which means that the div might only partially show up or not at all.

What's the simplest algorithm to tell how much % of the div is visible in the viewport?

(To make things easier, the div always fits into the viewport horizontally, so only the Y axis needs to be considered at the calculations.)

Answer 1

See one more example in fiddle: https://jsfiddle.net/1hfxom6h/3/

/*jslint browser: true*/
/*global jQuery, window, document*/
(function ($) {
    'use strict';
    var results = {};

    function display() {
        var resultString = '';

        $.each(results, function (key) {
            resultString += '(' + key + ': ' + Math.round(results[key]) + '%)';
        });

        $('p').text(resultString);
    }

    function calculateVisibilityForDiv(div$) {
        var windowHeight = $(window).height(),
            docScroll = $(document).scrollTop(),
            divPosition = div$.offset().top,
            divHeight = div$.height(),
            hiddenBefore = docScroll - divPosition,
            hiddenAfter = (divPosition + divHeight) - (docScroll + windowHeight);

        if ((docScroll > divPosition + divHeight) || (divPosition > docScroll + windowHeight)) {
            return 0;
        } else {
            var result = 100;

            if (hiddenBefore > 0) {
                result -= (hiddenBefore * 100) / divHeight;
            }

            if (hiddenAfter > 0) {
                result -= (hiddenAfter * 100) / divHeight;
            }

            return result;
        }
    }

    function calculateAndDisplayForAllDivs() {
        $('div').each(function () {
            var div$ = $(this);
            results[div$.attr('id')] = calculateVisibilityForDiv(div$);
        });

        display();
    }

    $(document).scroll(function () {
        calculateAndDisplayForAllDivs();
    });

    $(document).ready(function () {
        calculateAndDisplayForAllDivs();
    });
}(jQuery));

div {
    height:200px;
    width:300px;

    border-width:1px;
    border-style:solid;
}
p {
    position: fixed;
    left:320px;
    top:4px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="div1">div1</div>
<div id="div2">div2</div>
<div id="div3">div3</div>
<div id="div4">div4</div>
<p id="result"></p>