Modify width of rectangle based on page number in InDesign via Javascript

Question

I have a rectangle called `pageBar` on my master page !
I would like to create a script that will automatically resize the width of the rectangle based on the page number.
Something like : `width = 1280 * (pageNumber / pageTotal)`
Any help would be much appreciated.
Thanks in advance,
J.

Answer 1

Here's the script I ended up with. I'm a bit unhappy that I ended up having the literal string "pageBar" in it twice. The second instance is there to allow the script to be run on the same document twice -- that's one of the prices you pay for using master page items in a script rather than creating objects on the fly and applying an object style to them (better than setting selected properties):
//DESCRIPTION: Size PageBar on each page
(function() {
          if (app.documents.length > 0) {
                    sizePageBars(app.documents[0]);
          return;
function sizePageBars(aDoc) {
          var pageBar = getMasterItem(aDoc, "pageBar");
          if (pageBar == null) return;
          // walk through the document's pages and deal with each page
          var theMaster = pageBar.parent;
          var numPages = aDoc.pages.length;
          for (var j = numPages - 1; j >= 0; j--) {
                    if (theMaster == aDoc.pages[j].appliedMaster) { // ignore pages with other masters
                              processPage(aDoc.pages[j]);
          function processPage(page) {
                    // numPages, pageBar and j are visible as global variables in this function
                    // caculate desired width
                    var desiredHalfWidth = 1280 * (j + 1)/(numPages * 2); // j starts at zero;
                    // could be running script for a second time, so check to see if page already has pageBar on it
                    var bar = page.rectangles.item("pageBar");
                    if (bar == null) {
                              var bar = pageBar.override(page);
                    var pBounds = bar.geometricBounds;
                    var xCenter = (pBounds[3] + pBounds[1])/2;
                    pBounds[1] = xCenter - desiredHalfWidth;
                    pBounds[3] = xCenter + desiredHalfWidth;
                    bar.geometricBounds = pBounds;
          function getMasterItem(aDoc, name) {
                    var appliedMaster = aDoc.pages[0].appliedMaster;
                    var anItem = appliedMaster.rectangles.item(name);
                    if (anItem == null) {
                              alert("Couldn't locate 'pageBar' rectangle.");
                    return anItem;
I was going to introduce to the resize method, but that would have first meant understanding it myself. The description in the object model viewer is a masterful demonstration of how not to communicate complicated information.
So I did a web search and found myself on this page by Marc Autret: http://www.indiscripts.com/post/2013/05/indesign-scripting-forum-roundup-4
Search that page for resize and you'll see what I mean. Compared to the simple manipulation of geometric bounds, using resize is a nightmare of complexity. Marc has simplified it by providing a function you can call, but I decided to just go with using the bounds.
I hope this proves helpful to you.
Dave