setInterval对某个数值加加渐减

decrease_time = setInterval(decrease_opacity_val,10);
function decrease_opacity_val(){
    showID.style.opacity = showID.style.opacity - 0.01;
    if(showID.style.opacity == 0)
    {
        clearInterval(decrease_time);
        showID.style.display = "none";
        showID.style.opacity = "";

        leftID.style.display = "block";
        leftID.style.opacity = 0;

        increase_time = setInterval(increase_opacity_val,10);
    }
}

function increase_opacity_val(){
//加加需要使用parseFloat转换
    leftID.style.opacity = parseFloat(leftID.style.opacity) + 0.01;
    if(leftID.style.opacity == 1)
    {
        clearInterval(increase_time);
        leftID.style.opacity = "";
    }

}