Find duplicate IDs in a document.

dupefinder.js

(function () {
  "use strict";
  var find_dupes = function () {
    var identified = document.querySelectorAll('[id]'),
      found = {},
      id,
      idx;
    for (idx = 0; idx < identified.length; idx++) {
      id = (identified[idx]).getAttribute('id');
      if(found[id]) {
        found[id] += 1;
      } else {
        found[id] = 1;
      }
    }
    
    var dupes = [];
    for (id in found) {
      if(found[id] > 1) {
        dupes.push(id);
      }
    }
    
    if (dupes.length) {
      return "Duplicate IDs:" + dupes.join(", ");
    }
    return "No duplicate IDs";
  };
  console.log(find_dupes());
})();