function demoFun(){
var countryIdJS = document.getElementById("countryId").value;
var regionSelect = document.getElementById('regionId');
regionSelect.options.length = 0; // delete all existing options
regionSelect.options[0] = new Option ("--Select--","0");
// above line will add new option with value "0" and displayText "--Select--"
regionSelect.options[0].selected="true";
// above line will select the 0 th option