jquery 判断滚动方向 向上还是向下

scroll(function(direction) { console.log(direction) });
  
function scroll( fn ) {
    var $window = $(window),
        beforeScrollTop = $window.scrollTop(),
        fn = fn || function() {};

    $window.scroll(function() {
        var afterScrollTop = $window.scrollTop(),
            delta = afterScrollTop - beforeScrollTop;
        if( delta === 0 ) return false;
        fn( delta > 0 ? "down" : "up" );
        beforeScrollTop = afterScrollTop;
    });
}