451. Sort Characters By Frequency: Given a string, sort it in decreasing order based on the frequency of characters.

frequencySort.js

/**
 * @param {string} s
 * @return {string}
 */
var frequencySort = function(s) {
  var map = {};
  s.split('').forEach(function(char) {
    map[char] = map[char] ? map[char] + 1 : 1;
  });
  map = getEntries(map).sort(function(a, b) {
    return b[1] - a[1];
  });
  return map.reduce(function(ret, freq) {
    return ret + pad(freq);
  }, '');
};

/**
 * @param {object} object
 * @return {array} 2D-array of object key/value pairs
 */
function getEntries(object) {
  var entries = [];
  Object.keys(object).forEach(function(key) {
    entries.push([key, object[key]]);
  });
  return entries;
}

/**
 * @param {array} freq
 * @return {string}
 */
function pad(freq) {
  var padded = '';
  for (var i = 0; i < freq[1]; i++) {
    padded += freq[0];
  }
  return padded;
}

/**
 * @param {any} actual
 * @param {any} expected
 * @param {string} testName
 * @return {undefined}
 */
function assertEqual(actual, expected, testName) {
  if (actual === expected) {
    console.log('passed');
  } else {
    console.log('FAILED [' + testName + '] Expected "' + expected + '" but got "' + actual + '"');
  }
}

/**
 * @param {array} actual
 * @param {array} expected
 * @param {string} testName
 * @return {boolean}
 */
function assertDimensionalArraysEqual(actual, expected, testName) {
  if (Object.prototype.toString.call(actual) !== '[object Array]') {
    return actual === expected;
  }
  var isEqualLength = actual.length === expected.length;
  var isEqualItems = expected.every(function(item, index) {
    return assertDimensionalArraysEqual(item, actual[index], testName);
  });
  if (isEqualItems && isEqualLength) {
    console.log('passed');
    return true;
  } else {
    console.log('FAILED [' + testName + '] Expected "' + expected + '" but got "' + actual + '"');
    return false;
  }
}

var testVal = "cababa";
assertEqual(pad(['a', 3]), 'aaa', 'pads the character into a string');
assertDimensionalArraysEqual(
  getEntries({'a': 3, 'b': 2}), 
  [['a', 3], ['b', 2]], 
  'maps an object into a 2D array'
);
assertEqual(frequencySort(testVal), 'aaabbc', 'sorts a string');