var a = new Array(4);
alert(a[0]); // undefined
for (var i in a)
{
    alert("进不来！");
}

------解决方案--------------------
var   str   =   new   String("mm","bbb","ccc");

这个写法也是错的

------解决方案--------------------
<script   LANGUAGE="JavaScript">
var   arr   =   new   Array;
var   str   =   new   Array('aaa','bbb','ccc');
for(var   s   in   str){
//alert(s);
arr[str[s]]   =   str[s];
if   (arr[str[s]]   ==   "bbb"){
alert("aaa");
}
}  
</script>

------解决方案--------------------
aaa 出来了！
L@_@K
JScript code

var arr = new Array(); 
var str = new Array('aaa','bbb','ccc'); 
for (var s in str)
{  
    arr[s] = str[s];
    if (arr[s] == "bbb"){ 
        alert("aaa"); 
    } 
}