问用于检测项目是否由于滚动而不再可见的Javascript

EN

function test() {
    var $elem = $('.test');
    var visibleAtTop = $elem.offset().top + $elem.height() >= $(window).scrollTop();
    var visibleAtBottom = $elem.offset().top <= $(window).scrollTop() + $(window).height();
    if (visibleAtTop && visibleAtBottom) {
        alert('visible');
    } else {
        alert('invisible (at ' + (visibleAtTop ? 'bottom' : 'top') + ')');
    }
}