background.js

/*
  Actually we should filter URL here, but I am too lazy to do it.
  Let's just show this PageAction on ALL THE PAGES. (Yes this is stupid and should be avoided but whatever.)
*/
function checkForValidUrl(tabId, changeInfo, tab) {
  chrome.pageAction.show(tabId);
}

chrome.tabs.onUpdated.addListener(checkForValidUrl);

/*
  `content_script.js` and `background.js` use "Message Passing" to communicate with each other.
  FYI, `background.js` controls `page action`.

  Reference:
    https://developer.chrome.com/extensions/pageAction
    https://developer.chrome.com/extensions/tabs
    https://developer.chrome.com/extensions/messaging
*/
chrome.pageAction.onClicked.addListener(function(tab) {
  chrome.tabs.query({ active: true, currentWindow: true }, function(tabs) {
    chrome.tabs.sendMessage(
      tabs[0].id,
      { greeting: "onClickedPageActionButton" },
      function(response) {
        // do nothing
      }
    );
  });
});

/*
  Fired when the extension is first installed, when the extension is updated to a new version, and when Chrome is updated to a new version.
  Reference:
    https://developer.chrome.com/extensions/runtime
*/
chrome.runtime.onInstalled.addListener(function(details) {
  if (
    details.reason === chrome.runtime.OnInstalledReason.INSTALL ||
    details.reason === chrome.runtime.OnInstalledReason.UPDATE
  ) {
    // console.log("oninstalled or updated");
    chrome.tabs.create({ url: "updates.html" });
  }
});