How to Sort PDF bookmarks for free

// Copyright Sean Wingert, February 11, 2019
// Based largely on http://www.adobe.com/devnet/acrobat/pdfs/batch_sequences.pdf
// and https://wiki.med.umich.edu/display/UMHSHELPDESK/Acrobat+-+Using+Acrobat+JavaScript+to+Sort+PDF+Pages
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 3 of the License, or
// (at your option) any later version.
 
// Create a separator in the Edit menu and add a Sort Bookmarks entry. This loads a few moments after Acrobat does.
app.addMenuItem({cName:"-", cParent:"Edit", cExec:" "}); 
app.addMenuItem({cName:"Sort Bookmarks", cParent: "Edit", cExec: "freesort()"});
 
// Create a utility function to see if a PDF has been loaded first, which is a prerequisite
// https://coderwall.com/p/_g3x9q/how-to-check-if-javascript-object-is-empty
function isEmpty(obj) {
    for(var key in obj) {
        if(obj.hasOwnProperty(key))
            return false;
    }
    return true;
}
 
// Create a function to sort the parent bookmarks (not their children or grandchildren) by name
function freesort() {
if(isEmpty(this.bookmarkRoot)) { app.alert("Error. Please open a PDF first."); return; }
 
// set variables
bm = this.bookmarkRoot;  // the current PDF document
bmArray = new Array();   // array to hold collected bookmarks
 
// write bookmarks to an array because you can't sort this.bookmarkRoot.children.sort(compare)
for (var i=0; i < bm.children.length; i++) bmArray[i] = bm.children[i];
 
// Use Natural Sort, AKA Human Sort (1,2,11,22), not Alphabetical sort (1,11,2,22) 
// Note: toLowerCase() makes this a case-insensitive sort.
// https://stackoverflow.com/questions/4340227/sort-mixed-alpha-numeric-array
var reA = /[^~`!@#\$%\^&*()\-+=\[\]{}\\|;:'",<.>/?_a-zA-Z\s]/g;
var reN = /[^0-9]/g;
 
function compare(a, b) {
  var a=a.name.toLowerCase();
  var b=b.name.toLowerCase();
 
  var aA = a.replace(reA, "");
  var bA = b.replace(reA, "");
  if (aA === bA) {
    var aN = parseInt(a.replace(reN, ""), 10);
    var bN = parseInt(b.replace(reN, ""), 10);
    return aN === bN ? 0 : aN > bN ? 1 : -1;
  } else {
    return aA > bA ? 1 : -1;
  }
}
 
// sort the array
bmArray.sort(compare);
 
// Move the bookmarks to the end of the list in their sorted order
for (var i=0; i < bmArray.length; i++) bm.insertChild(bmArray[i], bm.children.length);
 
// troubleshooting: print the results
// for (var i=0; i < bmArray.length; i++) console.println(bmArray[i]);
}