The part: ....
document.getElementById('<%= lstFieldList_TrajectType(0)%>').value
needs to be dynamic.
The filelist is filled on another place in the aspx code-behind.
Because it is unknown how many elements will be in this list, I have to get the index dynamically.

This works: (say for 5 indexes)
document.getElementById('<%= lstFieldList_TrajectType(0)%>').value
document.getElementById('<%= lstFieldList_TrajectType(1)%>').value
document.getElementById('<%= lstFieldList_TrajectType(2)%>').value
document.getElementById('<%= lstFieldList_TrajectType(3)%>').value
document.getElementById('<%= lstFieldList_TrajectType(4)%>').value

So I need the index in lstFieldList_TrajectType(0) changed from lstFieldList_TrajectType(0) to something like lstFieldList_TrajectType(' + intCount + ').value
where intCout is the index that runs thru an while wend loop.

But when I try to put in this I get errors. I have no clue how to solve this.

Please advise.