GA custom event trigger which tracks reading behavior

read_vs_skim.js

<script>
try {
    document.addEventListener("DOMContentLoaded", function() {
        var accumScrollMove = 0;
        // Set it 1 msecs to avoid div by zero error
        var accumScrollTime = 1;
        var latestScrollTop = 0;
        var latestScrollAt = new Date();

        function checkScroll() {
            // 1. Check scrollbar movement speed
            var scrollTop = document.body.scrollTop;
            var scrollAt = new Date();
            var scrollTopDelta = Math.abs(latestScrollTop - scrollTop);
            var scrollTimeDelta = Math.min(scrollAt - latestScrollAt, 5 * 60 * 1000); // limit max time-delta to 5 minutes
            if(scrollTopDelta !== 0) {
                latestScrollTop = scrollTop;
                latestScrollAt = scrollAt;
                accumScrollMove += scrollTopDelta;
                accumScrollTime += scrollTimeDelta;
            }

            // 2. Check event firing condition
            if(accumScrollTime > 30 * 1000 && scrollTop / document.body.scrollHeight > 0.8) {
                var pixelPerSec = Math.floor(accumScrollMove / accumScrollTime * 1000);
                var label = pixelPerSec > 100 ? 'skimmer' : 'reader';
                ga('send', 'event', 'read', 'pps', label, pixelPerSec);
            } else {
                window.setTimeout(checkScroll, 500);
            }
        }

        checkScroll();
    });
} catch(e) {}
</script>