How does AngularJS implement change detection under the hood?

AngularJS uses a digest cycle with dirty checking to detect model changes and update the view.

Unlike modern frameworks using fine-grained reactivity, AngularJS relies on a digest cycle that iterates over all watchers registered on $scope. Each watcher compares previous and current values using dirty checking. If a change is detected, another digest cycle may be triggered until the system stabilizes. This recursive checking ensures consistency but can become expensive at scale due to O(n) watcher evaluation.