Count duplicated elements using Array.prototype.reduce

countDuplicatedElements.js

var arr = ['a','b','c','d','d','e','a','b','c','f','g','h','h','h','e','a'];
  return arr.reduce(function(acc, cur) {
    acc[cur] = (acc[cur] || 0) + 1;
    return acc;
  },{});
// prints { a: 3, b: 2, c: 2, d: 2, e: 2, f: 1, g: 1, h: 3 }