var arr=new Array('0');
i=0;
改为var arr=new Array('0')，i=0;

我式过了，确实和双引号等效，不过它在这里为什么要用单引号呢？
把整段代码贴上来

function formatno(n) {
    n = String(n);
    if (n.indexOf(".") != -1) {
        Num = n.substring(0, n.indexOf("."));
    } else {
        Num = n;
    }
    var arr = new Array('0'), i = 0;
    if (n.indexOf(",") == -1) {
        while (Num>0) {
            arr[i] = ''+Num%1000;
            Num = Math.floor(Num/1000);
            i++;
        }
        arr = arr.reverse();
        for (i in arr) {
            if (i>0) {
                while (arr[i].length<3) {
                    arr[i] = '0'+arr[i];
                }
            }
        }
    } else {
        trace("d") ;
        arr[i] = Num;
    }
    if (n.indexOf(".") != -1) {
        Dec = n.substring(n.indexOf(".")) ;
    } else {
        Dec = "";
    }
    if (Dec.length == 0 or Number(Dec == 0)) {
        arr += ".00";
    } else if (Dec.length == 2) {
        arr += Dec+"0";
    } else {
        arr += Dec;
    }
    delete Dec, Num, i;
    return arr;
}
var1 = 12345067.8;
var2 = formatno(var1);
trace(var2);

还是对单引号理解得比较抽象。