password.js

var Password = {},
    utils;

function getOptionNum(target, defaults, rest) {
  if (typeof(defaults) == 'undefined') defaults = 0;

  var val = (typeof(target) == 'undefined') ? defaults : target;

  if (val > rest) val = rest;

  return val;
}

function getRest(rest, quant) {
  var val = rest - quant;

  return (val < 0) ? 0 : val;
}

Password.generate = function(length, options) {
  var tmp  = [],
      defaults = Password.defaults,
      nums, specials, uppers, lowers;

  // Default values for arguments
  if (typeof(length) == 'undefined') length = defaults.length;
  if (length instanceof Array) length = utils.random(length[1], length[0]);
  options = options || {};

  // Assign quantity of different types
  specials = getOptionNum(options.specials, defaults.specials, length);
  length = getRest(length, specials);

  nums = getOptionNum(options.nums, defaults.nums, length);
  length = getRest(length, nums);

  uppers = getOptionNum(options.uppers, defaults.uppers, length);
  length = getRest(length, uppers);

  lowers = getOptionNum(options.lowers, defaults.lowers, length);
  length = getRest(length, lowers);

  // Make an array of symbols
  while(specials--) tmp.push(utils.special());
  while(nums--) tmp.push(utils.num());
  while(uppers--) tmp.push(utils.upperChar());
  while(lowers--) tmp.push(utils.lowerChar());
  while(length--) tmp.push(utils.anyChar());

  // Shuffle the array and make a string
  return utils.shuffle(tmp).join('');
}

Password.Utils = utils = {
  num: function(num) {
    return utils.random(9).toString();
  },

  upperChar: function() {
    return utils.charFrom(Password.charsUpperCase);
  },

  lowerChar: function() {
    return utils.charFrom(Password.charsLowerCase);
  },

  anyChar: function() {
    return utils.charFrom(Password.charsLowerCase + Password.charsUpperCase);
  },

  special: function() {
    return utils.charFrom(Password.specials);
  },

  charFrom: function(chars) {
    return chars.charAt(utils.random(chars.length - 1));
  },

  random: function(max, min) {
    if (typeof(min) == 'undefined') min = 0;

    return Math.floor(Math.random() * (max - min + 1) + min);
  },

  // Fisher-Yates shuffle
  shuffle: function(array) {
    var i = array.length;

    while (i--) {
      var j = Math.floor(Math.random() * (i + 1)),
          temp = array[i];

      array[i] = array[j];
      array[j] = temp;
    }

    return array;
  }
};

Password.specials = '~@#$%^&*()_-+=';
Password.charsLowerCase = 'abcdefghijklmnopqrstuvwxyz';
Password.charsUpperCase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
Password.defaults = {
  length: [7, 10],
  specials: 1,
  nums: 2,
  uppers: 2,
  lowers: 3
};