code: function createCombinations(a, b) {
    var combinations = [''];
    var newCombinations;
    for (var i = 0, len = a.length; i < len; i += 1) {
      newCombinations = [];
      combinations.forEach(function (combination) {
        newCombinations.push(combination + a[i]);
        // Don't repeat work.
        if (a[i] !== b[i]) {
          newCombinations.push(combination + b[i]);
        }
      });
      combinations = newCombinations;
    }
    return combinations;
  }
docs: a.length === b.length