$(window).scroll(function () {
		    var scrollH = window.screen.height;
		 
	        if ($(window).scrollTop() > scrollH) {
	            $("#mob_backTop").fadeIn(60);
//	            console.log($(window).scrollTop())
//	            console.log(window.innerHeight)
//	            console.log(scrollH)
//	            console.log(window.document.body.clientHeight)
//	            console.log(window.screen.availHeight)
	        }
	        else {
	            $("#mob_backTop").fadeOut(60);
	        }
    });
原文:http://www.cnblogs.com/zjpzjp/p/7295344.html