从无到右，从有到无的效果

$("#ul").css(‘opacity‘,0);
changeOpacity()；
//从无到有，2s；等待3s；从有到无，2s；等待1s；循环；
function changeOpacity() {
var waitTime=1000;
    $("#ul").fadeTo(2000,opacityUl); 
    if(opacityUl==1){
        opacityUl=0;
        waitTime=3000;
    }else if(opacityUl==0){
        opacityUl=1;
        waitTime=3000;
    }
    setTimeout(changeOpacity, waitTime);
}