效果见：http://cn.news.yahoo.com/
他的javascript代码如下，有达人指点一下，注释一下代码意思否？

<script language=javascript>
var isStart=true;
var nn;
var tt;
var bPlay=new Image;
bPlay.src = "http://cn.yimg.com/i/cn/con/06/bu_pla.gif";
var bPause=new Image;
bPause.src = "http://cn.yimg.com/i/cn/con/06/bu_pau.gif";
nn=1;
setTimeout('change_img()',4000);
function resetPlay(){
isStart=true;
var e = document.getElementById("top_slider");
var a = e.getElementsByTagName("img");
for(i=0;i if(a.src == bPlay.src) a.src = bPause.src;
}
}
function playorpau(e){
if(e.src == "http://cn.yimg.com/i/cn/con/06/bu_pau.gif"){
e.src = bPlay.src ;
isStart = false;
}else{
e.src = bPause.src;
isStart = true;
}
}
function pre_img(){
resetPlay();
nn--;
if(nn < 1) nn=5;
setFocus(nn);
}
function next_img(){
resetPlay();
nn++;
if(nn > 5) nn=1;
setFocus(nn);
}
function change_img()
{
if(isStart){
nn++;
if(nn>5) nn=1;
setFocus(nn);
}else{
tt=setTimeout('change_img()',100);
}
}
function setFocus(i)
{
if(tt) clearTimeout(tt);
nn = i;
selectLayer1(i);
tt=setTimeout('change_img()',4000);
}
function selectLayer1(i)
{
switch(i)
{
case 1:
document.getElementById("fnav1").style.display="block";
document.getElementById("enav1").style.display="none";
document.getElementById("snav1").style.display="none";
document.getElementById("cnav1").style.display="none";
document.getElementById("ynav1").style.display="none";
document.getElementById("f1").style.display="block";
document.getElementById("e1").style.display="none";
document.getElementById("s1").style.display="none";
document.getElementById("c1").style.display="none";
document.getElementById("y1").style.display="none";
break;
case 2:
document.getElementById("fnav1").style.display="none";
document.getElementById("enav1").style.display="block";
document.getElementById("snav1").style.display="none";
document.getElementById("cnav1").style.display="none";
document.getElementById("ynav1").style.display="none";
document.getElementById("f1").style.display="none";
document.getElementById("e1").style.display="block";
document.getElementById("s1").style.display="none";
document.getElementById("c1").style.display="none";
document.getElementById("y1").style.display="none";
break;
case 3:
document.getElementById("fnav1").style.display="none";
document.getElementById("enav1").style.display="none";
document.getElementById("snav1").style.display="block";
document.getElementById("cnav1").style.display="none";
document.getElementById("ynav1").style.display="none";
document.getElementById("f1").style.display="none";
document.getElementById("e1").style.display="none";
document.getElementById("s1").style.display="block";
document.getElementById("c1").style.display="none";
document.getElementById("y1").style.display="none";
break;
case 4:
document.getElementById("fnav1").style.display="none";
document.getElementById("enav1").style.display="none";
document.getElementById("snav1").style.display="none";
document.getElementById("cnav1").style.display="block";
document.getElementById("ynav1").style.display="none";
document.getElementById("f1").style.display="none";
document.getElementById("e1").style.display="none";
document.getElementById("s1").style.display="none";
document.getElementById("c1").style.display="block";
document.getElementById("y1").style.display="none";
break;
case 5:
document.getElementById("fnav1").style.display="none";
document.getElementById("enav1").style.display="none";
document.getElementById("snav1").style.display="none";
document.getElementById("cnav1").style.display="none";
document.getElementById("ynav1").style.display="block";
document.getElementById("f1").style.display="none";
document.getElementById("e1").style.display="none";
document.getElementById("s1").style.display="none";
document.getElementById("c1").style.display="none";
document.getElementById("y1").style.display="block";
break;
}
}
</script>
<script language="javascript">
function do_onload()
{
if(typeof(document.lgin)=='undefined' || (document.lgin.login.value=="" && document.lgin.passwd.value=="")) {
document.search.p.focus();
}
}
function do_crazy(){}
function do_wmad(){}
function do_ticket() {}
function do_all(){
do_crazy();
do_wmad();
do_ticket();
do_onload();
}
</script>
<script language=javascript>
function URLEncode(plaintext )
{
var SAFECHARS = "0123456789" + // Numeric
"ABCDEFGHIJKLMNOPQRSTUVWXYZ" + // Alphabetic
"abcdefghijklmnopqrstuvwxyz" +
"-_.!~*'()"; // RFC2396 Mark characters
var HEX = "0123456789ABCDEF";
var encoded = "";
for (var i = 0; i < plaintext.length; i++ ) {
var ch = plaintext.charAt(i);
if (ch == " ") {
encoded += "+"; // x-www-urlencoded, rather than %20
} else if (SAFECHARS.indexOf(ch) != -1) {
encoded += ch;
} else {
var charCode = ch.charCodeAt(0);
if (charCode > 255) {
encoded += "+";
} else {
encoded += "%";
encoded += HEX.charAt((charCode >> 4) & 0xF);
encoded += HEX.charAt(charCode & 0xF);
}
}
} // for
return encoded;
};
function rd_clk(path,ur) {
var newurl;
if (document.images){
ur = URLEncode(ur);
newurl="http://cn.rd.yahoo.com/home/portal/hp/"+ path + 'url='+ur+"/" + "*" + "http://cn.yimg.com/search/yisou/clk.gif";
(new Image()).src=newurl;
}
return true;
}
</script>
<script>
function setSrhFrm(selItem){
var frm;
var p;
var ac = selItem.value;
p = selItem.parentNode;
if(!p) return;
while(p.tagName.toLowerCase() !== 'form'){
p = p.parentNode;
}
p.action = ac;
}
</script>
<script language=javascript>
function URLEncode(plaintext )
{
var SAFECHARS = "0123456789" + // Numeric
"ABCDEFGHIJKLMNOPQRSTUVWXYZ" + // Alphabetic
"abcdefghijklmnopqrstuvwxyz" +
"-_.!~*'()"; // RFC2396 Mark characters
var HEX = "0123456789ABCDEF";
var encoded = "";
for (var i = 0; i < plaintext.length; i++ ) {
var ch = plaintext.charAt(i);
if (ch == " ") {
encoded += "+"; // x-www-urlencoded, rather than %20
} else if (SAFECHARS.indexOf(ch) != -1) {
encoded += ch;
} else {
var charCode = ch.charCodeAt(0);
if (charCode > 255) {
encoded += "+";
} else {
encoded += "%";
encoded += HEX.charAt((charCode >> 4) & 0xF);
encoded += HEX.charAt(charCode & 0xF);
}
}
} // for
return encoded;
};
function rd_clk(path,ur) {
var newurl;
if (document.images){
ur = URLEncode(ur);
newurl="http://cn.rd.yahoo.com/home/portal/hp/"+ path + 'url='+ur+"/" + "*" + "http://cn.yimg.com/search/yisou/clk.gif";
(new Image()).src=newurl;

}
return true;
}
</script>