jQuery 判断页面上下滚动

var t = 0,
    b = 0;
$(window).scroll(function(){
    t = $(this).scrollTop();
    if(b < t){
        console.log('向下滚动中...')
    }else{
        console.log('向上滚动中...')
    }
    setTimeout(function(){
        b = t
    }, 0)
})