判断网页上滚还是下滚

//判断是上滚还是下滚
function scroll(fn) {
    var beforeScrollTop = document.documentElement.scrollTop || document.body.scrollTop,
        fn = fn || function () {
            };
    window.addEventListener("scroll", function () {
        var afterScrollTop = document.documentElement.scrollTop || document.body.scrollTop,
            delta = afterScrollTop - beforeScrollTop;
        if (delta === 0) return false;
        fn(delta > 0 ? "down" : "up");
        beforeScrollTop = afterScrollTop;
    }, false);
}

scroll(function (direction) { console.log(direction) })