Get localized month name using native JS

var getMonth = function(idx) {

  var objDate = new Date();
  objDate.setDate(1);
  objDate.setMonth(idx-1);

  var locale = "en-us",
      month = objDate.toLocaleString(locale, { month: "long" });

    return month;
}

console.log(getMonth(1));
console.log(getMonth(12));