标题：js判断滚动条滚动方向
出处：刘新修
时间：Tue, 25 Oct 2016 10:09:51 +0000
作者：刘新修
地址：http://liuxinxiu.com:80/JS_scroll/

内容：
JavaScript代码
//第一种写法          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;                  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-----=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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