# What is NgZone service in Angular

What is **NgZone** service? How Angular is notified about the changes?
• Zone.js is one of the Angular dependencies which provide a mechanism, called zones, for encapsulating and intercepting asynchronous activities in the browser (e.g. setTimeout, setInterval, promises).
• These zones are execution contexts that allow Angular to track the start and completion of asynchronous activities and perform tasks as required (e.g. change detection). Zone.js provides a global zone that can be forked and extended to further encapsulate/isolate asynchronous behaviour, which Angular does so in its NgZone service, by creating a fork and extending it with its own behaviours.
Reactions