runtime.getBackgroundPage()

Retrieves the Window object for the background page running inside the current extension. If the background page is non-persistent (an event page) and it is not running, the background page is started.

This provides a convenient way for other privileged extension scripts to get direct access to the background script's scope. This enables them to access variables or call functions defined in that scope. "Privileged script" here includes scripts running in options pages, or scripts running in browser action or page action popups, but does not include content scripts.

Note that variables that were declared using const or let do not appear in the Window object returned by this function.

Also note that this method cannot be used in a private window in Firefox—it always returns null. For more info see related bug at bugzilla.

If the background page is an event page, the system will ensure it is loaded before resolving the promise.

This is an asynchronous function that returns a Promise.

let gettingPage = browser.runtime.getBackgroundPage()

// background.js

function foo() {
  console.log("I'm defined in background.js");
}

// popup.js

function onGot(page) {
  page.foo();
}

function onError(error) {
  console.log(`Error: ${error}`);
}

let getting = browser.runtime.getBackgroundPage();
getting.then(onGot, onError);