Handmade load balancer for Cloudflare Workers

index.js

const europe = ["AMS", "ATH", "BCN", "BEG", "TXL", "BRU", "OTP", "BUD", "KIV", "CPH", "DUB", "DUS", "EDI", "FRA", "GVA", "GOT", "HAM", "HEL", "IST", "KBP", "LIS", "LHR", "LUX", "MAD", "MAN", "MRS", "MXP", "DME", "MUC", "LCA", "OSL", "CDG", "PRG", "KEF", "RIX", "FCO", "LED", "SOF", "ARN", "TLL", "SKG", "VIE", "VNO", "WAW", "ZAG", "ZRH"];
const africa = ["CAI", "CMN", "CPT", "DAR", "JIB", "DUR", "JNB", "LOS", "LAD", "MPM", "MBA", "MRU", "RUN", "KGL"];
const asia = ["BKK", "CEB", "CTU", "MAA", "CMB", "SZX", "FUO", "FOC", "CAN", "HGH", "HAN", "HNY", "SGN", "HKG", "HYD", "ISB", "TNA", "KHI", "KTM", "KUL", "LHE", "NAY", "LYA", "MFM", "MNL", "BOM", "NNG", "DEL", "KIX", "PNH", "TAO", "ICN", "SHA", "SHE", "SJW", "SIN", "SZV", "TPE", "TSN", "NRT", "ULN", "WUH", "WUX", "XIY", "EVN", "CGO", "CSX"];
const latinAmerica = ["ASU", "BOG", "EZE", "CWB", "FOR", "LIM", "MDE", "MEX", "PTY", "POA", "UIO", "GIG", "GRU", "SCL", "CUR"];
const middleEast = ["AMM", "BGW", "GYD", "BEY", "DOH", "DXB", "KWI", "BAH", "MCT", "ZDM", "RUH", "TLV"];
const northAmerica = ["IAD", "ATL", "BOS", "BUF", "YYC", "CLT", "ORD", "CMH", "DFW", "DEN", "DTW", "IAH", "IND", "JAX", "MCI", "LAS", "LAX", "MFE", "MEM", "MIA", "MSP", "MGM", "YUL", "BNA", "EWR", "ORF", "OMA", "PHX", "PIT", "PDX", "QRO", "RIC", "SMF", "SLC", "SAN", "SJC", "YXE", "SEA", "STL", "TPA", "YYZ", "YVR", "TLH", "YWG"];
const oceania = ["AKL", "BNE", "MEL", "PER", "SYD"]

const euroApi = "europe-west1-XXXXXX.cloudfunctions.net";
const usApi = "us-central1-XXXXXX.cloudfunctions.net";
const asiaApi = "asia-east2-XXXXXX.cloudfunctions.net";

addEventListener('fetch', event => {
  // A promise is required
  event.respondWith(eventHandler(event))
})

async function eventHandler(event){
  let hostname = matchGeo(event.request.cf ? event.request.cf.colo : "BRU") // the "cf" object is not accessible in the Playground
  return await fetch(genRequest(event.request, hostname)).then((response) => {
    if (response.status < 400) // If OK or redirect
      return response;
    else throw new Error(response.status + " " + hostname);
  }).catch(async (err) => {
    // In case of issues, try another host
    console.error(err);
    return await fetch(genRequest(event.request, getBackupHost(hostname))).then((response) => {
      console.log(response);
      if (response.status < 400) // if OK or redirect
        return response;
      else throw new Error(response.status);
    }).catch((err) => {
      console.error(err)
      return new Response(null, {status: 500}); // We're giving up
    });
  });
}

function matchGeo(colo) {
  if (europe.includes(colo))
    return euroApi;
  else if (africa.includes(colo))
    return euroApi;
  else if (asia.includes(colo))
    return asiaApi;
  else if (latinAmerica.includes(colo))
    return usApi;
  else if (middleEast.includes(colo))
    return euroApi;
  else if (northAmerica.includes(colo))
    return usApi;
  else
    return asiaApi;
}

function getBackupHost(hostname) {
  switch(hostname){
    case usApi:
      return euroApi;
    default: 
      return usApi;
  }
}

function genRequest(request, hostname) {
  let url = new URL(request.url)
  url.hostname = hostname
  return new Request(url, request)
}