Checking an HTML page for duplicate IDs using JavaScript

duplicateIDCheckerJavaScript

var allElements = document.getElementsByTagName("*");
var allIds = {};
var found = false;
for (var i = 0, n = allElements.length; i < n; ++i) {
  var id = allElements[i].id;
  if (id) {
    if (allIds[id] === undefined) {
      allIds[id] = 1;
    } else {
      found = true;
      console.warn('Duplicate ID #' + id);
    }
  }
}
if (!found) {
  console.log('No duplicate IDs found');
}