Find duplicate IDs in your DOM

With a dynamic application with lots of shared templates it can sometimes be easy to use the same ID for two different elements on the same page. Sometimes this can manifest itself by your JS not working quite right but it isn’t apparent what the problem is. Here is a quick script you can paste into your console to report any duplicates.

var allElements = document.getElementsByTagName("*"), allIds = {}, dupIDs = [];
for (var i = 0, n = allElements.length; i < n; ++i) {
  var el = allElements[i];
  if (el.id) {
  	if (allIds[el.id] !== undefined) dupIDs.push(el.id);
  	allIds[el.id] = el.name || el.id;
    }
  }
if (dupIDs.length) { console.error("Duplicate ID's:", dupIDs);} else { console.log("No Duplicates Detected"); }

Math Busche covered the same topic last year, head over there to see how he tackled the same thing.