Error on running the JScript to write data on Excel

https://support.smartbear.com/testcomplete/docs/testing-with/advanced/working-with-external-data-sources/excel/working-via-com.html

Please check how properly operate with Excel object.

Also this topic can helps: https://msdn.microsoft.com/en-us/library/wss56bz7.aspx

Your issue is here:

var book = app.Workbooks.Open(fname);

app.Workbooks.Open() changes the state of app object, but does not return anything.