sort indicator

I am using server side sorting to sort the data in the grid. But I am unable to set the sort indicator properly. First time when I clicked on the Header, I got empty for alert(obj.getSortDirection(index)). So, I used the following code to set the indicators.

if(obj.getSortDirection(index) == '' || obj.getSortDirection(index) == 'descending')
{
obj.setSortDirection('ascending',index);
}else if(obj.getSortDirection(index) == 'ascending') {
obj.setSortDirection('descending',index);
}

But I am able to get the indicator only in one direction, because alert(obj.getSortDirection(index)) always give me blank. Please help me.