disconnected

In less than 0.5K, it enables connected and disconnected element's listeners in hyperHTML, but it can also be used with any other library/vanilla JS.

The only optional dependencies it has are constructable Event and the WeakSet. Both must be passed along as configuration object, and polyfills might be needed only for legacy browsers.

// requires both modern Event and WeakSet
import disconnected from 'disconnected';
const observe = disconnected({Event, WeakSet});

observe(mainElement);
mainElement.addEventListener('connected', () => {
  console.log('I am alive ;-)');
});
mainElement.addEventListener('disconnected', () => {
  console.log('but now I am out O_O');
});

observe(subElement);
observe(topElement);
observe(anyElement);

Compatibility

Even IE9, as long as a usable Event and WeakSet are provided.

What about attributechanged ?

You got it, sharing same API, needing same Event poly, if necessary.

DOM Level 0 Like events ?

Using with-level-0 would make it possible to have el.onconnected = ... simplification too.

withLevel0('connected');
withLevel0('disconnected');

// remember to observe the node
var div = observe(document.createElement('div'));

// add your Level 0 listener
div.onconnected = function () {
  div.textContent = 'Level 0';
};

// that's it!
document.body.appendChild(div);

// feel free to clean it up via
div.onconnected = null;