jQuery实现 返回顶部

//若出现滚动条，且超出30px则，出现那个图片
var st = jQuery(document).scrollTop(),winh = jQuery(window).height;
var topImg = document.getElementById("topImg");
window.onscroll = function(){
    st = jQuery(document).scrollTop(),winh = jQuery(window).height();
    if(st > 30){
        topImg.style.display = "inline";
        topImg.style.top = st + winh -150 +"px";
    }else{
        topImg.style.display = "none";
    }
}

//点击那图片，回到顶部
function clickTop(){
    jQuery("html,body").animate({scrollTop:0},120);
    topImg.style.display = "none";
}