background.js

chrome.runtime.onInstalled.addListener(function () {
  // Replace all rules ...
  chrome.declarativeContent.onPageChanged.removeRules(undefined, function () {
    // With a new rule ...
    chrome.declarativeContent.onPageChanged.addRules([
      {
        // That fires when a page's URL contains a 'g' ...
        conditions: [
          new chrome.declarativeContent.PageStateMatcher({
            pageUrl: { hostContains: "burpple", urlContains: "timeline" },
          }),
        ],
        // And shows the extension's page action.
        actions: [new chrome.declarativeContent.ShowPageAction()],
      },
    ]);
  });
});

chrome.runtime.onMessage.addListener((msg, sender) => {
  // First, validate the message's structure.
  if (msg.from === "content" && msg.subject === "init") {
    // Enable the page-action for the requesting tab.
    console.log("content script initialised");
  }
  if (msg.from === "content" && msg.subject === "viewMore") {
    // Enable the page-action for the requesting tab.
    console.log("view more");
  }
});