Mutation Observer
A web API that notifies when the DOM changes — nodes added/removed, attributes changed, text updated. It gives an efficient, asynchronous way to react to changes in the page's structure, even ones you didn't cause yourself.
The old problem: the old "mutation events" were synchronous and fired for every little change → catastrophic performance, and they were deprecated. Mutation Observer does it right: you register which types of changes you care about (child nodes, attributes, text, subtree) on an element, and the browser batches the changes and calls your callback asynchronously → efficient even with many changes. Use case: react to DOM you don't control (a third-party widget, user-generated content), detect when an element you're waiting for appears, synchronize something with dynamic changes, build developer tools. Win: reliable, performance-friendly DOM monitoring. The third observer API (with Intersection and Resize). Used more rarely in app code (frameworks usually handle DOM changes themselves) but indispensable for libraries/tools. Related to intersection observer and resize observer.