var finalEnglishToBanglaNumber = {
   0: "০",
   1: "১",
   2: "২",
   3: "৩",
   4: "৪",
   5: "৫",
   6: "৬",
   7: "৭",
   8: "৮",
   9: "৯",
};

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 : ১২৩৪৫৬

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

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