How to find duplicate id's in a form?

Question

I\'ve created a form with about 800 fields in it. Unknowingly I\'ve given same id for few fields in the form. How to trace them?

Answer 1

One-ish liner using just array methods:

[].map.call(document.querySelectorAll("[id]"), 
     function (e) {
        return e.id;
     }).filter(function(e,i,a) {
        return ((a.lastIndexOf(e) !== i) && !console.log(e));
     })

Logs every duplicate and returns an array containing the ids if any were found.