code: function(str, num) {
      if (isUndefined(num)) num = 1;
      var start = str.length - num < 0 ? 0 : str.length - num;
      return str.substr(start);
    }
docs: *
  @method last([n] = 1)
  @returns String
  @short Returns the last [n] characters of the string.
  
  @example
  
  'lucky charms'.last()  -> 's'
  'lucky charms'.last(3) -> 'rms'
  
  @param {number} [n]
  
  *