Event when window.location.href changes

I use this script in my extension "Grab Any Media" and work fine ( like youtube case )

var oldHref = document.location.href;

window.onload = function() {
    var bodyList = document.querySelector("body")

    var observer = new MutationObserver(function(mutations) {
        mutations.forEach(function(mutation) {
            if (oldHref != document.location.href) {
                oldHref = document.location.href;
                /* Changed ! your code here */
            }
        });
    });
    
    var config = {
        childList: true,
        subtree: true
    };
    
    observer.observe(bodyList, config);
};

You can't avoid polling, there isn't any event for href change.

Using intervals is quite light anyways if you don't go overboard. Checking the href every 50ms or so will not have any significant effect on performance if you're worried about that.

popstate event:

The popstate event is fired when the active history entry changes. [...] The popstate event is only triggered by doing a browser action such as a click on the back button (or calling history.back() in JavaScript)

So, listening to popstate event and sending a popstate event when using history.pushState() should be enough to take action on href change:

window.addEventListener('popstate', listener);

const pushUrl = (href) => {
  history.pushState({}, '', href);
  window.dispatchEvent(new Event('popstate'));
};