How to GBK(GB2312) and UTF-8 encoding conversion between in JavaScript

following js code show to how to convert text between Simplified Chinese GB 2312 text strings into UTF8 using JavaScript.

GB2312UTF8  = {
  Dig2Dec : function(s){
      var retV = 0;
      if(s.length == 4){
          for(var i = 0; i < 4; i ++){
              retV += eval(s.charAt(i)) * Math.pow(2, 3 - i);
          }
          return retV;
      }
      return -1;
  } ,

  Hex2Utf8 : function(s){
     var retS = "";
     var tempS = "";
     var ss = "";
     if(s.length == 16){
         tempS = "1110" + s.substring(0, 4);
         tempS += "10" +  s.substring(4, 10);
         tempS += "10" + s.substring(10,16);
         var sss = "0123456789ABCDEF";
         for(var i = 0; i < 3; i ++){
            retS += "%";
            ss = tempS.substring(i * 8, (eval(i)+1)*8);
            retS += sss.charAt(this.Dig2Dec(ss.substring(0,4)));
            retS += sss.charAt(this.Dig2Dec(ss.substring(4,8)));
         }
         return retS;
     }
     return "";
  } ,

  Dec2Dig : function(n1){
      var s = "";
      var n2 = 0;
      for(var i = 0; i < 4; i++){
         n2 = Math.pow(2,3 - i);
         if(n1 >= n2){
            s += '1';
            n1 = n1 - n2;
          }
         else
          s += '0';
      }
      return s;
  },

  Str2Hex : function(s){
      var c = "";
      var n;
      var ss = "0123456789ABCDEF";
      var digS = "";
      for(var i = 0; i < s.length; i ++){
         c = s.charAt(i);
         n = ss.indexOf(c);
         digS += this.Dec2Dig(eval(n));
      }
      return digS;
  },

  GB2312ToUTF8 : function(s1){
    var s = escape(s1);
    var sa = s.split("%");
    var retV ="";
    if(sa[0] != ""){
      retV = sa[0];
    }
    for(var i = 1; i < sa.length; i ++){
      if(sa[i].substring(0,1) == "u"){
        //alert(this.Str2Hex(sa[i].substring(1,5)));
        retV += this.Hex2Utf8(this.Str2Hex(sa[i].substring(1,5)));
  if(sa[i].length){
    retV += sa[i].substring(5);
  }
      }
      else{
     retV += unescape("%" + sa[i]);
  if(sa[i].length){
    retV += sa[i].substring(5);
  }
   }
    }
    return retV;
  },

  UTF8ToGB2312 : function(str1){
        var substr = "";
        var a = "";
        var b = "";
        var c = "";
        var i = -1;
        i = str1.indexOf("%");
        if(i==-1){
          return str1;
        }
        while(i!= -1){
    if(i<3){
                substr = substr + str1.substr(0,i-1);
                str1 = str1.substr(i+1,str1.length-i);
                a = str1.substr(0,2);
                str1 = str1.substr(2,str1.length - 2);
                if(parseInt("0x" + a) & 0x80 == 0){
                  substr = substr + String.fromCharCode(parseInt("0x" + a));
                }
                else if(parseInt("0x" + a) & 0xE0 == 0xC0){ //two byte
                        b = str1.substr(1,2);
                        str1 = str1.substr(3,str1.length - 3);
                        var widechar = (parseInt("0x" + a) & 0x1F) << 6;
                        widechar = widechar | (parseInt("0x" + b) & 0x3F);
                        substr = substr + String.fromCharCode(widechar);
                }
                else{
                        b = str1.substr(1,2);
                        str1 = str1.substr(3,str1.length - 3);
                        c = str1.substr(1,2);
                        str1 = str1.substr(3,str1.length - 3);
                        var widechar = (parseInt("0x" + a) & 0x0F) << 12;
                        widechar = widechar | ((parseInt("0x" + b) & 0x3F) << 6);
                        widechar = widechar | (parseInt("0x" + c) & 0x3F);
                        substr = substr + String.fromCharCode(widechar);
                }
     }
     else {
      substr = substr + str1.substring(0,i);
      str1= str1.substring(i);
     }
              i = str1.indexOf("%");
        }

        return substr+str1;
  }
};

test:
  GBK => UTF8:
  var utf8 = GB2312UTF8.GB2312ToUTF8("中文GB2312");

  UTF8 => GBK:
  GB2312UTF8.UTF8ToGB2312(utf8);