javascript中indexOf()和lastIndexOf()方法用法

<script language="JavaScript" type="text/javascript">
 
// 说明：Javascript Array 的 lastIndexOf() 方法

 
if (!Array.prototype.lastIndexOf)
{
    Array.prototype.lastIndexOf = function(elt /*, from*/)
    {
        var len = this.length;
 
        var from = Number(arguments[1]);
        if (isNaN(from))
        {
            from = len - 1;
        }
        else
        {
            from = (from < 0)
                ? Math.ceil(from)
                : Math.floor(from);
            if (from < 0)
                from += len;
            else if (from >= len)
                from = len - 1;
        }
 
        for (; from > -1; from--)
        {
            if (from in this && this[from] === elt)
                return from;
        }
        return -1;
    };
}
</script>