Find elements with duplicated ID

findDuplicatedIDs.js

(function(field, scope) {
  var id,
      allIds = {},
      elementsWithId = scope.querySelectorAll('[' + field + ']');

  Array.prototype.forEach.call(elementsWithId, function(elem) {
    if(field.substr(0,5) === 'data-') {
      id = elem.dataset[field.substr('5')];
    } else {
      id = elem[field];
    }
    allIds[id] = (allIds[id] || 0) + 1;
  });

  // remove non-duplicated elements from object filled before
  for(id in allIds) {
    if(allIds[id] < 2) {
      delete allIds[id];
    }
  }

  // output
  if( JSON.stringify(allIds) === "{}" ) {
    console.log('No duplicated IDs in this document.');
    return false;
  } else {
    return allIds;
  }

})('id', document);

//use data-reactid for react components