Feb 22 2016 Find Duplicate Element in JavaScript Array 1234567function uniq(l) { return l.filter(function (e, i) { return l.indexOf(e) === i; });}function findDup(l) { return uniq(l.filter(function (e, i) { return l.indexOf(e) !== i; }));} Example:12> findDup([1, 2, 3, 4, 2, 5, 1, 2]);< [2, 1]