20 Angular Change Detection Interview Questions and Answers

Angular is a popular web development framework used by many developers to create single-page web applications. Angular’s change detection feature allows developers to automatically detect and update data bindings when the underlying data changes. This can be a useful tool for developers who need to manage large and complex data sets.

If you’re interviewing for a position that involves working with Angular, it’s likely that the interviewer will ask you questions about change detection. In this article, we’ll review some of the most common questions about Angular change detection and how you should answer them.

Angular Change Detection Interview Questions and Answers

Here are 20 commonly asked Angular Change Detection interview questions and answers to prepare you for your interview:

1. What is Angular Change Detection?

Angular Change Detection is a system that automatically checks for changes in the component tree and makes the corresponding changes to the DOM. This process is triggered whenever a user interacts with the component, either through an event or a property binding.

2. Can you explain how Angular uses data binding to update the view automatically whenever a change occurs in the model?

Angular uses data binding to keep the view and the model in sync. Whenever a change occurs in the model, Angular will update the view to reflect that change. This is done automatically, so that the user always sees the most up-to-date information.

3. How does angular detect changes in the model?

Angular uses something called dirty checking to detect changes in the model. This means that it will compare the current value of a model to the previous value to see if there has been any change. If there is a change, then Angular will update the view accordingly.

4. Is it possible to use zone.js to trigger change detection manually? If yes, then can you give an example of doing that?

Yes, it is possible to use zone.js to trigger change detection manually. You can do this by using the NgZone.run() function. For example:

“`
import { Component, NgZone } from ‘@angular/core’;

@Component({
selector: ‘my-app’,
template: `

`,
})
export class AppComponent {
message: string;

constructor(private _ngZone: NgZone) {}

ngOnInit() {
this.message = ‘Hello world!’;

this._ngZone.run(() => {
// do something
});
}
}
“`

5. What are some common scenarios where Change Detection fails to work properly?

There are a few different reasons why Change Detection might fail to work as intended. One common reason is if the developer accidentally mutates an object in the component’s scope instead of creating a new object. This can cause Change Detection to get confused and not realize that anything has changed. Another reason is if the developer is using an Observable library that doesn’t properly trigger Change Detection. This can cause the component to not update even when the data has changed.

6. What’s the difference between ZoneAwarePromise and Promise?

ZoneAwarePromise is a promise that will run its callback functions in the Angular zone, which is necessary for Angular change detection to occur. Promise, on the other hand, will run its callback functions outside of the Angular zone. This can be useful if you want to avoid triggering Angular change detection, but it means that you will have to manually trigger it yourself if you want to see any changes that occur inside of the promise callback.

7. What are some common pitfalls with asynchronous programming in JavaScript?

There are a few common pitfalls when it comes to asynchronous programming in JavaScript. One is that if you try to update a variable that is being used in an asynchronous callback, the callback may not have the updated value. Another is that if you have a chain of asynchronous callbacks, it can be difficult to keep track of the order in which they will be executed. Finally, if an asynchronous callback throws an error, it can be difficult to track down where the error occurred.

8. What do you understand by zones in context with Angular?

Zones are a mechanism used by Angular to keep track of changes that happen in the app. By default, Angular will run change detection on every component every time something in the app changes. This can be expensive, so Angular allows you to create zones and specify which components should be checked when something in the app changes. This way, you can control when and how often change detection is run, which can help improve performance.

9. Why do we need zones in Angular?

Zones are necessary in Angular because they provide a way to intercept and keep track of asynchronous tasks that are happening outside of the Angular framework. By doing this, Angular can automatically trigger change detection when it detects that an asynchronous task has completed, which ensures that the component is always up-to-date.

10. What is the purpose of the ngZone service?

The ngZone service is used to run a function inside of Angular’s change detection zone. This is useful for ensuring that Angular will pick up on changes that have been made outside of the Angular framework.

11. Where can the ngZone service be injected?

The ngZone service can be injected into any Angular component or service. This service is responsible for managing change detection in Angular applications. By default, Angular runs change detection on all components every time an event occurs. This can be expensive, and can cause performance problems in large applications. The ngZone service allows developers to control when and how often change detection is run. This can help improve performance in large applications.

12. What is the default change detection strategy used in Angular?

The default change detection strategy used in Angular is called “dirty checking”. This means that when the change detector runs, it will check all of the bindings in the component to see if they have changed. If any of them have changed, then the component will be updated accordingly.

13. How can you disable change detection for a specific component or directive?

You can disable change detection for a specific component or directive by setting the changeDetection property of the component or directive to ChangeDetectionStrategy.OnPush.

14. Can you describe the process of executing change detection in Angular?

Angular change detection is a process that is used to keep track of changes in the data bound to an Angular application. This process starts when the Angular application is bootstrapped, and then runs in the background in order to check for changes. When a change is detected, Angular will then update the appropriate parts of the application accordingly.

15. Can you list some of the applications where using setTimeout() might lead to unexpected results?

There are a few potential issues that can arise from using setTimeout() in Angular applications. One is that if you are using setTimeout() to delay a change in your model, the change may not be picked up by Angular’s change detection mechanism. This can lead to unexpected results, especially if you are relying on the model change to trigger some other behavior in your application. Another potential issue is that if you are using setTimeout() to schedule a function to run after a certain amount of time, the function may not run at the exact time you expect it to. This is because setTimeout() is not a guaranteed timer – it may run slightly earlier or later than the specified time, depending on the browser and other factors.

16. Can you explain what microtasks and macrotasks mean?

Microtasks are tasks that are executed after the current task has finished. They are typically used for cleaning up after an asynchronous operation, and they are handled by the browser’s event loop. Macrotasks, on the other hand, are tasks that are executed after the current task has finished, but they are handled by the browser’s UI thread.

17. How do web workers affect change detection in Angular?

Web workers are able to run JavaScript code in the background, independently of the main UI thread. This can be used to perform heavy computations or tasks that might otherwise block the UI. However, it also means that any changes that are made in the web worker will not be detected by Angular’s change detection mechanism. This can lead to unexpected behavior if the web worker modifies data that is being used by the UI.

18. What is OnPush change detection?

OnPush change detection is a strategy used in Angular to limit when change detection is run. By default, Angular will run change detection on every component every time something changes in the app. This can be expensive, and so OnPush change detection limits it to only run when an input binding changes, or when an event fires from the component.

19. What is the significance of markForCheck() method in context with change detection?

The markForCheck() method is used to mark a component for change detection. This is useful if you need to make sure that a component is checked even if it is not actively being used. For example, if you have a component that is only used when a user clicks a button, you can use markForCheck() to make sure that the component is checked when the button is clicked.

20. Can you explain the difference between runOutsideAngular() and run() methods on NgZone? Which one would you prefer in certain situations?

The runOutsideAngular() method allows you to run a function outside of the Angular zone. This can be useful if you need to avoid triggering Angular change detection, or if you need to run a long-running task that doesn’t need to be registered with Angular. The run() method runs a function inside the Angular zone. This is the default behavior for Angular, and is generally what you want. However, there are some cases where you may need to run a function inside the Angular zone, such as if you need to trigger Angular change detection.