popup.js

chrome.tabs.query(
  { active: true, windowId: chrome.windows.WINDOW_ID_CURRENT },
  function(tabs) {
    const { id: tabId } = tabs[0].url;

    let code = `document.querySelector('h1')`;

    // http://infoheap.com/chrome-extension-tutorial-access-dom/
    chrome.tabs.executeScript(tabId, { code }, function (result) {
      // result has the return value from `code`
    });
  }
);