src/common/cs_event_listener.js

// In order to pass i18n strings and settings values to the injected scripts,
// which don't have access to the chrome.* APIs, we use event listeners.
window.addEventListener('TWPT_sendRequest', evt => {
  var request = evt.detail;
  switch (request.data.action) {
    case 'geti18nMessage':
      var data = chrome.i18n.getMessage(
          request.data.msg,
          (Array.isArray(request.data.placeholders) ?
               request.data.placeholders :
               []));
      break;

    case 'getProfileIndicatorOptions':
      var data = {
        'indicatorDot': options.profileindicator,
        'numPosts': options.profileindicatoralt
      };
      break;

    case 'getNumPostMonths':
      var data = options.profileindicatoralt_months;
      break;

    default:
      var data = 'unknownAction';
      console.warn('Unknown action ' + request.data.action + '.');
      break;
  }

  var response = {
    data,
    requestId: request.id,
    prefix: (request.prefix || 'TWPT'),
  };

  window.postMessage(response, '*');
});