Javascript var overrides pipeline var


I’m trying to pass a value which is assigned from a select list into a flow service in following javascript function:
function enableTextarea()
%scope param(keyPair=‘initValue’)%
keyPair = document.frm1.fileLoad.value;
%invoke Test.propertyTest:getFileContentAsStr%
document.frm1.inString.value = “%value configStr%”;

service getFileContentAsStr
input var: keyPair
output var: configStr

How do I assign real value (document.frm1.fileLoad.value) to pipeline var: keyPair? the code I posted that always return “initValue” string.