Page Scroll Progress With JavaScript

scrollProgress.js

// Read more about it here: https://lab.moin.im/2015/03/18/page-scroll-progress/

function scrollProgress (el, updateFunc, endFunc) {
	document.addEventListener('DOMContentLoaded', function() {
		var progress = 0,
		end = false,
		timeStart = null,
		timeEnd = null,
		height = 0,
		elt = null;
		
		if( el == window ) {
			el = document.body;
			elt = window;
			height = window.innerHeight;
		} else {
			el = document.getElementById(el);
			elt = el;
			height = el.clientHeight;
		}

	    elt.addEventListener('scroll', function() {
	        if(timeStart == null)
	            timeStart = Date.now();

	        progress = (el.scrollTop / ( el.scrollHeight - height ) ) * 100;

	        updateFunc( progress );

	        if(progress >= 100 && !end) {
	            timeEnd = Date.now();
	            end = true;
	            endFunc( timeEnd - timeStart );
	        }
	    });
	});
}