sortCharactersByFrequency.js

/**
 * @param {string} s
 * @return {string}
 */
var frequencySort = function(s) {
    var characters = {};
    for (var i = 0; i < s.length; i++) {
        if (s[i] in characters) {
            characters[s[i]]++;
        } else {
            characters[s[i]] = 1;
        }
    }
    var sortable = [];
    for (var k in characters) {
      sortable.push([k, characters[k]]);
    }
    var sorted = sortable.sort(function (a, b) {
        return b[1] - a[1];
    })
    var string = '';

    for (var j = 0; j < sorted.length; j++) {
      var character = sorted[j][0]
      for (var l = 0; l < sorted[j][1]; l++) {
        string += character;
      }
    }
    return string;
};