Convert from legacy to perm thread id

conversion.js

// background.js

// memoize thread ids
const threadIds = {};

// register redirect handler to capture perm thread ids
function memoizePermThreadId(details) {
  const originalUrl = new URL(details.url);
  if (originalUrl.searchParams.has('idr')) {
      var legacyThreadId = decodeURIComponent(originalUrl.searchParams.get('idr')).split('/')[1];
      var redirectUrl = new URL(details.redirectUrl);
      var permThreadId = decodeURIComponent(redirectUrl.hash.split('/')[1]).substring(1); // cut off leading hash #
      threadIds[legacyThreadId] = permThreadId;
  }
}

chrome.webRequest.onBeforeRedirect.addListener(
  memoizePermThreadId,
  {
    urls: ['https://mail.google.com/*'], // filter: ajax requests to gmail
    types: ['xmlhttprequest']
  }, 
  ['responseHeaders']
);

// register message handler to get perm thread id from legacy thread id
function fetchPermThreadId(request, sender, sendResponse) {
    if (request.message === 'gmail::thread::fetch_perm_thread_id') {
      if (threadIds.hasOwnProperty(request.legacyThreadId) {
          sendResponse(threadIds[request.legacyThreadId]);
      } else {
        var gmailUrl = new URL(sender.url); // important this comes from Gmail for multi-user accounts
        var permThreadIdUrl = new URL(gmailUrl.protocol + gmailUrl.hostname + gmailUrl.pathname);
        permThreadIdUrl.searchParams.set('idr', 'all/' + request.legacyThreadId); // using `all` label 
        fetch(permThreadIdUrl, { method: 'HEAD', mode: 'cors' })
          .then(function() { sendResponse(threadIds[request.legacyThreadId]); }); // by this point, onBeforeRequest has memoized the perm thread id
      }
    }
    return true; // async
}

chrome.runtime.onMessage.addListener(fetchPermThreadId);

// gmail.js
function navigateToThread(legacyThreadId) {
  chrome.runtime.sendMessage(
    { message: 'gmail::thread::fetch_perm_thread_id', legacyThreadId }, // must be hex, not decimal
    function(permThreadId) {
      window.location.hash = '#all/' + permThreadId;
    } 
  );  
}