example chrome message passing (background)

background.js

chrome.runtime.onMessage.addListener(
  function(request, sender, sendResponse) {
    // listen for messages
    if (request.type === 'fromContent') {
      // use own func to process (or just store directly)
      let data = processData(request.data);
      localStorage.setItem(request.key, JSON.stringify(data));
    } else if (request.type == 'fromPopup') {
      let data = JSON.parse(localStorage.getItem(request.key));
      sendResponse({
        data: data
      });
    }
  
  // should be included by default
  return true;
  }
);