scroll.js

(function() {
	var delay = 1000 / 12;
	var last = +new Date();
	var step = 25;
	var interval = {};
	
	var getScrollProgress = function() {
		return (document.body.scrollTop / (document.body.scrollHeight - window.innerHeight)) * 100;
	};
	
	var report = function(progress) {
		console.log(progress);
	};
	
	var track = function() {
		var now = +new Date();
		
		var isComplete = interval['100'];
		var tooSoon = now - last < delay;

		if (isComplete || tooSoon) return;
			
		last = now;
		
		var progress = getScrollProgress();
		
		for (var i = step; i <= 100; i += step) {
			if (!interval[i] && progress >= i) {
				interval[i] = true;
				report(i);
			}			
		}
	}
	
	window.addEventListener('scroll', track);
}());