content.js

const useChrome = typeof (browser) === 'undefined';

function handleMessage (message) {
  if (message.getStorageData && message.getStorageData === true) {
    const localStorageData = {};
    const sessionStorageData = {};

    for (const key of Object.keys(window.localStorage)) {
      localStorageData[key] = window.localStorage.getItem(key);
    }

    for (const key of Object.keys(window.sessionStorage)) {
      sessionStorageData[key] = window.localStorage.getItem(key);
    }

    if (useChrome) {
      chrome.runtime.sendMessage({ local: localStorageData, session: sessionStorageData });
    } else {
      browser.runtime.sendMessage({ local: localStorageData, session: sessionStorageData });
    }

    return;
  }

  if (message.clearStorage) {
    if (message.clearStorage === 'local') {
      window.localStorage.clear();
    } else if (message.clearStorage === 'session') {
      window.sessionStorage.clear();
    }

    storageWindowHandler(null);
  }
}

function storageWindowHandler () {
  handleMessage({ getStorageData: true });
}

if (useChrome) {
  chrome.runtime.onMessage.addListener(handleMessage);
} else {
  browser.runtime.onMessage.addListener(handleMessage);
}

window.addEventListener('storage', storageWindowHandler);