https://dasmedacademy.pl/js/addons/counter.js

1// counter
2if (document.querySelector('.counter')) {
3	var counters = document.querySelectorAll('.counter');
4	var c = document.querySelector('.counter');
5	var counted;
6
7	var isInViewport = function (elem) {
8		var distance = elem.getBoundingClientRect();
9		return (
10			distance.top >= 0 &&
11			distance.left >= 0 &&
12			distance.bottom <=
13				(window.innerHeight || document.documentElement.clientHeight) &&
14			distance.right <=
15				(window.innerWidth || document.documentElement.clientWidth)
16		);
17	};
18
19	window.addEventListener(
20		'scroll',
21		function (event) {
22			if (isInViewport(c)) {
23				countUp(counters, 1000);
24				counted = true;
25			} else {
26				counted = false;
27			}
28		},
29		false
30	);
31
32	function countUp(element, interval) {
33		Array.prototype.slice.call(element).forEach(function (counter) {
34			var max = counter.innerHTML;
35			var i = 0;
36			counter.innerHTML = i;
37			var timer = interval / max;
38
39			if (!counted) {
40				var counting = setInterval(function () {
41					i += 1;
42					if (i <= max) {
43						counter.innerHTML = i;
44					} else {
45						clearInterval(counting);
46					}
47				}, timer);
48			} else {
49				counter.innerHTML = max;
50			}
51		});
52	}
53}