Javascript Array find even Occurence

function evenOccurence (arr) {
  var occurenceMap = {};
  for(var i = 0; i < arr.length; i++){
    occurenceMap[arr[i]]++ || (occurenceMap[arr[i]] = 1)
  }//ww w  . ja  va  2  s  . com

  for(var i = 0; i < arr.length; i++){
    if(occurenceMap[arr[i]] % 2 === 0){
      return arr[i];
    }
  }

  return 'no even occurrences'
}

// tests
var arr1 = [ 1, 3, 3, 3, 2, 4, 4, 2, 5 ]
console.log(evenOccurence(arr1));