JSFiddle - React, Tailwind, and code Playground

JavaScript

var charMap = {
    "ā" : "a",
    "ū" : "u"
};
var region="Ahmadābād,Sūrat,Vadodara,Rājkot,Bhāvnagar,Jāmnagar,Nadiād,Gāndhīnagar,Jūnāgadh,Surendranagar";

var chars = region.split('');
var charsConverted = [];

for(var i = 0; i < chars.length; i++){
    var char = chars[i];
    charsConverted.push( charMap[char] || char);
}

var result = charsConverted.join('');

alert(region);
alert(result);