<SCRIPT language=JavaScript>
var imgUrl=new Array();
var adNum=0;
imgUrl[1]="http://a.thec.cn/maojunjin/images/kkkk_1.gif";
imgUrl[2]="http://a.thec.cn/maojunjin/images/kkkk_2.gif";
imgUrl[3]="http://a.thec.cn/maojunjin/images/kkkk_3.gif";
imgUrl[4]="http://a.thec.cn/maojunjin/images/kkkk_4.gif";
imgUrl[5]="http://a.thec.cn/maojunjin/images/kkkk_5.gif";
var imgPre=new Array();
var j=0;
for (i=1;i<=5;i++) {
    if((imgUrl[i]!="")){j++;} 
    else {break;}
    }
function playTran(){
    if(document.all)imgInit.filters.revealTrans.play();
    }
var key=0;
function nextAd(){
    if(adNum<j)adNum++;
    else adNum=1;
    if( key==0 ){key=1;} 
    else if (document.all){
             imgInit.filters.revealTrans.Transition=24;
             imgInit.filters.revealTrans.apply();
             }    document.images.imgInit.src=imgUrl[adNum];
    playTran();
    theTimer=setTimeout("nextAd()", 3500);
    }
</SCRIPT>
这段代码  function playTran(){
    if(document.all)imgInit.filters.revealTrans.play();
    }
中的if(document.all)是什么意思 啊,该怎么理解它。请高手赐教