为什么智能实现opacity的替换

// JavaScript Document

function startmove(obj,attr,iTarget,fn){

clearInterval(obj.timer);

obj.timer=setInterval(function(){

//取当前的值

var icur=0;

if(attr=='opacity'){

icur=Math.round(parseFloat(getStyle(obj,attr))*100);

}

else{

icur=parseInt(getStyle(obj,attr));

}

//2算速度

var speed=(iTarget-icur)/8;

speed=speed>0?Math.ceil(speed):Math.floor(speed);

//3.检测停止

if(icur==iTarget){

clearInterval(obj.timer);

if(fn){

fn();}

}

else{ 

if(attr='opacity'){

obj.style.filter='alpha(opacity:'+(icur+speed)+')';

obj.style.opacity=(icur+speed)/100;

}else{

obj.style[attr]=icur+speed+'px';

}

}

},30)

}

function getStyle(obj,attr){

if(obj.currentStyle){

return obj.currentStyle[attr];

}

else{

return getComputedStyle(obj,false)[attr];

}

}