var finalEnglishToBanglaNumber = {
   0: "",
   1: "",
   2: "",
   3: "",
   4: "",
   5: "",
   6: "",
   7: "",
   8: "",
   9: "",
};

String.prototype.getDigitBanglaFromEnglish = function () {
   var retStr = this;
   for (var x in finalEnglishToBanglaNumber) {
      retStr = retStr.replace(
         new RegExp(x, "g"),
         finalEnglishToBanglaNumber[x]
      );
   }
   return retStr;
};

var english_number = "123456";

var bangla_converted_number = english_number.getDigitBanglaFromEnglish();

//outputs : 

console.log(bangla_converted_number); //or alert(bangla_converted_number);

// another reference
// https://stackoverflow.com/questions/30629242/javascript-replace-to-english-number