count.js

function countFrequency(arr) {
  return arr.reduce(function(result, ele){
    // Judge whether this element has been counted before
    if (result.get(ele) != undefined) {
      /**
        * If this element has been counted before,
        * increase the frequency of its occurrence by 1
        */
      result.set(ele, result.get(ele) + 1)
    } else {
      /**
        * If this element has not been counted before,
        * set the frequency of its occurrence to 1
        */
      result.set(ele, 1);
    }
    return result;
  }, new Map());
}