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 = "";
	}

}