Find duplicate IDs in a document.

dupefinder.js

(function () {
  "use strict";
  var find_dupes = function () {
    var identified = document.querySelectorAll('[id]'),
      unique = [],
      dupes = [],
      id,
      idx = 0;
    while (idx < identified.length) {
      id = (identified[idx]).getAttribute('id');
      if (/^\s+$/.test(id)) {
        id = "WHITESPACE";
      }
      if (/^$/.test(id)) {
        id = "EMPTYSTRING";
      }
      if (unique.indexOf(id) === -1) {
        unique.push(id);
      } else {
        if (dupes.indexOf(id) === -1) {
          dupes.push(id)
        }
      }
      idx++;
    }
    if (dupes.length) {
      return dupes.length + " Duplicate IDs found:\n" + dupes.join("\n");
    }
    return "No duplicate IDs";
    };
  console.log(find_dupes());
})();