JavaScript implementation of Base58

Base58.js

/**
  This is a javascript port of the Base58 code present at
  http://icoloma.blogspot.com.es/2010/03/create-your-own-bitly-using-base58.html
*/
var Base58 = (function() {
    
    var 
      BASE58_CHARS = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";

    return {
      numberToAlpha: function(number) {
        var buffer = '';
        do {
          buffer = buffer + BASE58_CHARS[number % BASE58_CHARS.length];
         number = (number / BASE58_CHARS.length) >> 0;
        } while (number > 0);
        return buffer;
      },

      alphaToNumber: function(chars) {
        var result = 0, multiplier = 1, index = 0;
        for (; index < chars.length; index++) {
           var c = chars[index];
           var digit;
           if (c >= '1' && c <= '9') {
             digit = c - '1'; 
           } else if (c >= 'A' && c < 'I') {
             digit = (c - 'A') + 9;
           } else if (c > 'I' && c < 'O') {
             digit = (c - 'J') + 17;
           } else if (c > 'O' && c <= 'Z') {
             digit = (c - 'P') + 22;
           } else if (c >= 'a' && c < 'l') {
              digit = (c - 'a') + 33;
           } else if (c > 'l' && c <= 'z') {
             digit = (c - 'l') + 43;
           } else {
              throw new Error("Illegal character found: '" + c + "'");
           }
            
           result += digit * multiplier;
           multiplier = multiplier * BASE58_CHARS.length;
        }
        return result;
      }
    };

})();

// Uncomment one of these to use as bookmarklet
alert("The equivalent number is: " + Base58.alphaToNumber(prompt("Introduce a Base58 text")));
alert("The Base58 conversion is: " + Base58.numberToAlpha(prompt("Introduce a number", 0)));