js判断滚动条滚动方向

来源：互联网发布：票务管理系统源码编辑：程序博客网时间：2024/06/06 02:37
//第一种写法        var a;                   function scroll( fn ) {            var beforeScrollTop = document.documentElement.scrollTop ||window.pageYOffset|| document.body.scrollTop,                fn = fn || function() {};            window.addEventListener("scroll", function() {                var afterScrollTop = document.documentElement.scrollTop ||window.pageYOffset|| 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);            a =  direction;        });         // 第二种写法        var a;                   function scroll( fn ) {            var beforeScrollTop = document.body.scrollTop,                fn = fn || function() {};            window.addEventListener("scroll", function() {                var afterScrollTop = document.body.scrollTop,                    delta = afterScrollTop - beforeScrollTop;                if( delta === 0 ) return false;                a = fn( delta > 0 ? "down" : "up" );                beforeScrollTop = afterScrollTop;            }, false);        }        scroll(function(direction) {             // console.log(direction);            return direction;        });         // 第三种写法------这样写不知道fn这个函数的用意,那样写很明白就清楚fn的意思了，是执行完事件后的回调函数        var a;        var beforeScrollTop = document.body.scrollTop,            fn = fn || function() {};        window.addEventListener("scroll", function() {            var afterScrollTop = document.body.scrollTop,                delta = afterScrollTop - beforeScrollTop;            if( delta === 0 ) return false;            fn( delta > 0 ? "down" : "up" );            beforeScrollTop = afterScrollTop;        }, false);        function fn(direction) {             // console.log(direction);            a = direction;        };        window.onscroll=function(){            console.log(a);        }
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