Format dates with pattern matching in Javascript

getTimestamp.js

const monthNames = [
    "January", "February", "March",
    "April", "May", "June",
    "July", "August", "September",
    "October", "November", "December"
];

const timeZone = n => ({
  true: "Asia/Jakarta", 
  [ n === "WIB" ]: "Asia/Jakarta",
  [ n === "WITA" ]: "Asia/Makassar",
  [ n === "WIT" ]: "Asia/Jayapura",
}).true

function getTimestamp(timezone, plainDate = new Date()) {
  const dateString = plainDate.toLocaleString("en-US", {
    timeZone: timeZone(timezone),
  });
  
  const dateObject = new Date(dateString);
  
  const dd = String(dateObject.getDate()).padStart(2, '0');
  const mm = monthNames[dateObject.getMonth()].substr(0, 3);
  const yyyy = dateObject.getFullYear();
  const hh = String(dateObject.getHours()).padStart(2, '0');
  const MM = String(dateObject.getMinutes()).padStart(2, '0');
  
  const formattedTimestamp = mm + " " + dd + " " + yyyy + " " + hh + ":" + MM + ":" + "00";
  return formattedTimestamp;
}

console.log(getTimestamp("WIB"))