(function (window, undefined) {

	if (!window.localStorage) {
		if (window.console) {
			console.info('xybookmark cannot access localStorage');
			return;
		}
	}


	var LS = window.localStorage;

	var xyb = {
		lsname : 'LS_XYB',
		list : [],
		load : function () {
			this.list = LS.getItem(this.lsname) ? JSON.parse(LS.getItem(this.lsname)) : [];
		},
		find : function (url) {
			for (var i = 0, ll = this.list.length; i < ll; i++) {
				var entry = this.list[i];
				if (entry.url === url) {
					return i;
				}
			}
			return -1;
		},
		save : function (url) {
			var title = document.getElementsByTagName('title')[0].innerHTML;
			this.list.push({
				title : title,
				url : url,
				timestamp : Date.now(),
				scrollY : window.scrollY,
				scrollX : window.scrollX
			});
		},
		commit : function () {
			LS.setItem(this.lsname, JSON.stringify(this.list));
		},
		onWindowKeyDown : function (e) {
			if (e.keyCode === 66 /*m*/
				 && e.shiftKey && e.ctrlKey)
				this.bookmark();
		},
		scrollTo : function () {
			var idx = this.find(window.location.href);
			if (idx > -1) {
				var item = this.list[idx];
				window.scrollTo(item.scrollX, item.scrollY);
			}
		},
		bookmark : function () {
			var url = window.location.href;
			var idx = this.find(url);
			if (idx > -1) {
				this.list[idx].scrollX = window.scrollX;
				this.list[idx].scrollY = window.scrollY;
			} else {
				this.save(url);
			}
			this.commit();
		},
		remove : function () {
			var idx = this.find(window.location.href);
			if (idx > -1) {
				this.list.splice(idx, 1);
			}
			this.commit();
		},
		reset : function () {
			this.list = [];
			LS.removeItem(this.lsname);
		}
	};

	xyb.load();
	xyb.scrollTo();
	window.addEventListener('keydown', xyb.onWindowKeyDown.bind(xyb));

	window.XYB = window.XYB || {
		addBookmark : xyb.bookmark.bind(xyb),
		removeBookmark : xyb.remove.bind(xyb),
		reset : xyb.reset.bind(xyb)
	};

})(window);

https://github.com/andreasi/xybookmark

xybookmark is a lightweight JavaScript library that allows the readers of a blog to boomark the scroll position of a long article.
More specifically it saves the scrollX and scrollY state of a page so that when the user revisits the url, the page scrolls down or right to the position that was bookmarked. xybookmark uses localStorage to save the urls of a website

Handling scrolling of long articles on a blog

JavaScript