A zone is an execution context that persists across async tasks. You can think of it as [thread-local storage](http://en.wikipedia.org/wiki/Thread-local_storage) for JavaScript VMs.
This guide describes how to use Angular's NgZone to automatically detect changes in the component to update HTML.
## Fundamentals of change detection
To understand the benefits of `NgZone`, it is important to have a clear grasp of what change detection is and how it works.
### Displaying and updating data in Angular
In Angular, you can [display data](guide/displaying-data) by binding controls in an HTML template to the properties of an Angular component.
In addition, you can bind DOM events to a method of an Angular component. In such methods, you can also update a property of the Angular component, which updates the corresponding data displayed in the template.
After you update the data, you need to call `detectChange()` manually to check whether the data changed.
If the data changed, you render the HTML to reflect the updated data.
In Angular, this step is unnecessary. Whenever you update the data, your HTML is updated automatically.
### When apps update HTML
To understand how change detection works, first consider when the application needs to update the HTML. Typically, updates occur for one of the following reasons:
1. Component initialization. For example, when bootstrapping an Angular application, Angular loads the bootstrap component and triggers the [ApplicationRef.tick()](api/core/ApplicationRef#tick) to call change detection and View Rendering. Just as in the [displaying data](guide/displaying-data) sample, the `AppComponent` is the bootstrap component. This component has the properties `title` and `myHero`, which the application renders in the HTML.
2. Event listener. The DOM event listener can update the data in an Angular component and also trigger change detection, as in the following example.
4. MacroTasks, such as `setTimeout()` or `setInterval()`. You can also update the data in the callback function of a `macroTask` such as `setTimeout()`. For example:
5. MicroTasks, such as `Promise.then()`. Other asynchronous APIs return a Promise object (such as `fetch`), so the `then()` callback function can also update the data. For example:
6. Other async operations. In addition to `addEventListener()`, `setTimeout()` and `Promise.then()`, there are other operations that can update the data asynchronously. Some examples include `WebSocket.onmessage()` and `Canvas.toBlob()`.
The preceding list contains most common scenarios in which the application might change the data. Angular runs change detection whenever it detects that data could have changed.
The result of change detection is that the DOM is updated with new data. Angular detects the changes in different ways. For component initialization, Angular calls change detection explicitly. For [asynchronous operations](https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Asynchronous), Angular uses a zone to detect changes in places where the data could have possibly mutated and it runs change detection automatically.
A zone provides an execution context that persists across async tasks. [Execution Context](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this) is an abstract concept that holds information about the environment within the current code being executed. Consider the following example:
In the following example, the new zone context is called `zoneThis`.
```javascript
zone.run(() => {
// now you are in a zone
expect(zoneThis).toBe(zone);
setTimeout(function() {
// the zoneThis context will be the same zone
// when the setTimeout is scheduled
expect(zoneThis).toBe(zone);
});
});
```
This new context, `zoneThis`, can be retrieved from the `setTimeout()` callback function, and this context is the same when the `setTimeout()` is scheduled.
To get the context, you can call [`Zone.current`](https://github.com/angular/angular/blob/master/packages/zone.js/lib/zone.ts).
-`onHasTask`: triggers when the status of one kind of task inside a zone changes from stable to unstable or from unstable to stable. A status of "stable" means there are no tasks inside the zone, while "unstable" means a new task is scheduled in the zone.
* Zone JS is required by default for Angular itself.
*/
import 'zone.js/dist/zone'; // Included with Angular CLI.
```
Before importing the `zone.js` package, you can set the following configurations:
- You can disable some asynchronous API monkey patching for better performance.
For example, you can disable the `requestAnimationFrame()` monkey patch, so the callback of `requestAnimationFrame()` will not trigger change detection.
This is useful if, in your application, the callback of the `requestAnimationFrame()` will not update any data.
- You can specify that certain DOM events do not run inside the angular zone; for example, to prevent a `mousemove` or `scroll` event to trigger change detection.
By default, `Zone` is loaded and works without additional configuration. However, you don't necessarily have to use `Zone` to make Angular work. Instead, you can opt to trigger change detection on your own.
**If you disable `Zone`, you will need to trigger all change detection at the correct timing yourself, which requires comprehensive knowledge of change detection**.
