Intersection observer with detection of vertical scroll direction.

IntersectionObserverWithDirection.js

function createIntersectionObserver(callback, opts = { root: null, rootMargin: '0px', threshold: 0}) {
    var previousY = new Map();
  
    var observer = new IntersectionObserver(function(entries, observer){
        console.log(entries);
        entries.forEach(function (entry) {
            var currY = entry.boundingClientRect.y;
            var prevY = previousY.get(entry.target);
            if(currY<prevY) { /* scroll down */ entry.scrollDirectionY = 'down'; }
            if(currY>prevY) { /* scroll up */ entry.scrollDirectionY = 'up'; }
            callback(entry, observer, entries);
            previousY.set(entry.target, currY);
        });
    }, opts)
  
    return observer;
}
var observer = createIntersectionObserver(function(entry){
    if (entry.isIntersecting) {
        entry.target.classList.add("a-show");
    }
    else {
        if(entry.scrollDirectionY == 'up') { 
            entry.target.classList.remove("a-show");
        }
    }
});
  
document.querySelectorAll('.table-list tr').forEach(function(el){ observer.observe(el); })