How to write into excel file using Javascript?

In JavaScript (not JScript) you need to access Excel collection items by using the .Items property:

book.Sheets(sheetName) => book.Sheets.Item(sheetName)
app.Cells(1,1) => app.Cells.Item(1,1)

etc.

More details here: https://support.smartbear.com/viewarticle/82954/#collections

Also, you can't assign to a cell directly because it's an object, you need to use something like:

app.Cells.Item(1,1).Value2 = "test";