script to insert new master on every other page

I am not sure where I originally got this script but I have been using it for years.  It works perfectly for inserting the Master (Parent) Page of your choice after every other page in a document - for adding a backside to a multi-up mail mergeed letter or postcard, for example.  For those who are new to scripts (as I was) just copy the text below and past into Notepad (PC) or Textedit (Mac) then choose Save As and make a text document on your desktop with the extension .js  It should show up with the S JavaScript icon.  Then open InDesign, open scripts, scroll down to the user folder, right click on it and choose "Reveal in Explorer" or the Mac equaivalent, then drag the script into that folder.  It will show up in your user scriipt menu without having to restart InDesign.  Hope it works for you!

var doc = app.documents[0];
var masterNames = doc.masterSpreads.everyItem().name;
var d = app.dialogs.add({name:"pick a master spread"});
d.dialogColumns.add().staticTexts.add({staticLabel:"Master Pages:"});
var dd = d.dialogColumns.add().dropdowns.add({stringList:masterNames});
if(d.show()){
var index = dd.selectedIndex;
d.destroy();
} else {
d.destroy();exit();
}
var master = doc.masterSpreads.item(index);
for(var i=doc.pages.length-1;i>=0;i--){
doc.pages.add(LocationOptions.AFTER,doc.pages[i],{appliedMaster:master});}