Javascript Rotation Cipher

rot_cipher.js

/*
  Rotation Ciphers
*/

var RotCipher = (function() {
  
  var alpha = "abcdefghijklmnopqrstuvwxyz";
  
  var obj  = {
    result    : "",
    char      : "",
    pos       : 0,
    rotate    : 0,
    
    run : function decode(msg, rot) {

      if (rot >= 26 || rot < 1) {
        console.error('use a rotation less than 26, and greater than 1!');
        return false;
      }

      var self = this;
      this.rotate = rot;
      
      for (i = 0; i < msg.length; i++) {
        self.char = msg.charAt(i);
        self.getPosition().replaceChar();
      }
      
      return this.result;
    },
    
    getPosition : function getPosition() {
      this.pos = alpha.indexOf(this.char.toLowerCase()) - this.rotate;
      return this;
    },
    
    replaceChar : function replaceChar() {
        var ch = this.char;
        var abc = (this.isUpperCase(ch)) ? alpha.toUpperCase() : alpha;

        if (ch === " " || this.isNumber(ch) || this.isPunc(ch) ) {
          this.result += ch;
          return this;
        }
        
        if (this.pos < 0) {
          this.result += abc.charAt(26+this.pos);
        } else {
          this.result += abc.charAt(this.pos);
        }
        
        return this;
     
    },
    
    isNumber : function isNumber( char ) {
      if ( typeof char !== "undefined" && !isNaN(parseInt( char)) ) {
        return true;
      } else if ( !isNaN(parseFloat( char)) ) {
        return true;
      }
        
      return false;
    },
    
    isPunc : function isPunc( char ) {
      var puncReg = /[\u2000-\u206F\u2E00-\u2E7F\\'!"#\$%&\(\)\*\+,\-\.\/:;<=>\?@\[\]\^_`\{\|\}~]/g;
      if (puncReg.test(char)) {
        return true;
      }
      
      return false;
    },

    isUpperCase : function usUpperCase( char ) {
      if ( char < 'a' ) {
        return true;
      }

      return false;
    }
    
  };
  
  return obj;
  
})();

var msg = RotCipher.run("Url zna, vz va gbja naq qbja ng gur cho univat svfu naq puvcf naq n orre. Whfg purpxvat lbh fgvyy unir gung frperg xrl v tnir lbh lrnef ntb! nyfb, vgf gvzr v gbyq lbh gur gehgu. Vz gur mbqvnp xvyyre, qbag gryy naq ab whqtvrf x?", 13);
console.log(msg);

This rotates the given text by any number passed in as the second argument, so it can be used to encode and decode a simple rotation cipher, it handles uppercase letters, ignores both integers and floats, as well as punctuation.