Redux middleware sending messages to add-on on state changes

index.js

const postMessage = (type, data = {}) =>
  window.postMessage(
    { ...data, type, channel: `${CHANNEL_NAME}-extension` },
    "*"
  );

const updateExtensionThemeMiddleware = ({ getState }) => next => action => {
  const returnValue = next(action);
  const meta = action.meta || {};
  if (!meta.skipAddon && themeChangeActions.includes(action.type)) {
    postMessage("setTheme", { theme: selectors.theme(getState()) });
  }
  return returnValue;
};