onscroll.js

(function() {
	var timer = 0;
	var screenViewport =  window.innerHeight;
	var documentHeight = document.documentElement.scrollHeight;
	var scrollPositions = {
		0: 0,
		25: parseInt((documentHeight / 100) * 25, 10),
		50: parseInt((documentHeight / 100) * 50, 10),
		75: parseInt((documentHeight / 100) * 75, 10),
		100: parseInt(documentHeight - (screenViewport / 2), 10)
	};
    var scrollTags = {
        0: false,
        25: false,
        50: false,
        75: false,
        100: false
    };
	window.addEventListener('scroll', function() {
		clearTimeout(timer);
		timer = setTimeout(function() {
			calculatePageScroll();
		}, 300);
	});
	function calculatePageScroll() {
		var _height = documentHeight - screenViewport;
		var _scrollTop = window.scrollY;
		var _scrollPercent = parseInt((_scrollTop * 100) / _height);
		trackScrollPercent(_scrollPercent);
	}
	function trackScrollPercent(tag_percent) {
		for (var percent in scrollTags) {
			if ((percent <= tag_percent) && (scrollTags[percent] === false)) {

				if (scrollTags[percent] !== true) {
					console.log('scroll:percentage', percent + 'p');
				}
				scrollTags[percent] = true;
			}
		}
	}
	calculatePageScroll()
})()