JS - Save File as Field Name in the Data Merge Source File

Hi All - I need some help...

I'm trying to figure out the JavaScript code to save an indd file using the data merge source file field value.

As an example...

Let's say I have an invoice template called template.indd that is linked to invoice.txt.

To make it easy, let's say the text file has 1 field called "invoice number" with a value of 123

I run data merge.

Now I want to save the merged indd file as 123.indd

I've got everything figured out in JS except getting the source text file data into a variable to use when saving.  This one's been giving me fits for hours!

Is this possible??

Thanks very much