common_letters.js

// Write a function to get the common letters out of two strings 
// commonLetters('hello world', 'hello moon') => should return 'helo'

function commonLetters(str1, str2) {
  var map = {},
      common = '';

  for (var i = 0; i < str1.length; i++) {
    var letter = str1[i];
    if (letter != ' ') map[letter] = true;
  }

  for (var i = 0; i < str2.length; i++) {
    var letter = str2[i];
    if (map[letter] && common.indexOf(letter) == -1) common += letter;
  }

  return common;
}