text: // Background script
  // - handles clicks on the extension icon
  // - coordinates communication between content scripts
  
  // forwards all messages from content scripts
  chrome.runtime.onMessage.addListener((request, sender, sendResponse) => {
    // TODO: pass along response
    sendMessage(request);
  });
  
  // broadcasts when the browser action is clicked
  chrome.browserAction.onClicked.addListener(tab => {
    sendMessage({
      command: "browser action",
    });
  });
  
  // abstract away minutiae of message sending
  function sendMessage(message, callback) {
    chrome.tabs.query({
      active: true,
      currentWindow: true,
    }, tabs => {
      const { id } = tabs[0];
      chrome.tabs.sendMessage(id, message, callback);
    });
  }
  