Sorts a string by the frequency of characters.

sort_by_letter_freq.js

var sort_by_freq = function(str) {
  var i, j;
  var map = [];
  var sorted = [];
  for (i = 0; i < str.length; i++) {
    if (map[str.charAt(i)] == null) {
      map[str.charAt(i)] = 1;
    } else {
      map[str.charAt(i)] = map[str.charAt(i)] + 1;
    }
  }
  for (j in map) {
    sorted.push([j, map[j]]);
  }
  sorted.sort(function(a,b) { return b[1] - a[1] });
  return sorted;
}