Html javascript code that check the duplicate id

Question

How to check the tags that contains duplicate ids in javascript?

Answer 1

Try this:

var nodes = document.querySelectorAll('[id]');
var ids = {};
var totalNodes = nodes.length;

for(var i=0; i<totalNodes; i++) {
    var currentId = nodes[i].id ? nodes[i].id : "undefined";
    if(isNaN(ids[currentId])) {
        ids[currentId] = 0;
    }                 
    ids[currentId]++;
}

console.log(ids);

http://jsfiddle.net/sekVp/1

Answer 2

Here is a shorter way to achieve the same as the approved answer (count occurrences of IDs):

const ids = Array.from(document.querySelectorAll('[id]'))
  .map(v => v.id)
  .reduce((acc, v) => { acc[v] = (acc[v] || 0) + 1; return acc }, {});
console.log(ids);

If only list of duplicate IDs is needed, the entries can be filtered:

Object.entries(ids)
  .filter(([key, value]) => value > 1)
  .map(([ key, value]) => key)