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 = "";

    }

}