20 Angular Reactive Forms Interview Questions and Answers

Angular Reactive Forms provide a model-driven approach to handling form inputs whose values can change over time. This approach can be used for any type of form, including complex forms. When interviewing for a position that involves Angular Reactive Forms, you may be asked questions about your experience with this technology. In this article, we review some common Angular Reactive Forms interview questions and provide guidance on how to answer them.

Angular Reactive Forms Interview Questions and Answers

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

1. What is Angular Reactive Form?

Angular Reactive Form is a form building library for Angular that makes it easy to create reactive forms. A reactive form is one that automatically updates the form data when the underlying data changes. This can be useful for things like keeping a form synchronized with a database.

2. Can you explain the main difference between Template-Driven and Reactive Forms in Angular?

The main difference between the two types of forms is that template-driven forms are more concerned with the structure of the form, while reactive forms are more concerned with the behavior of the form. With template-driven forms, you are responsible for creating the entire form in the template, including all of the input fields, validation, and submit handling. With reactive forms, on the other hand, you create the form in the component class, and then bind it to the template. This gives you more control over the form, but it can also be more difficult to set up.

3. What are some advantages of using Reactive forms over template-driven forms?

Reactive forms offer more flexibility and control over form validation. With template-driven forms, validation is typically done through directives, which can be difficult to control. Reactive forms also allow for more complex validation, such as cross-field validation.

4. How do you create a reactive form in Angular?

You can create a reactive form in Angular by using the FormBuilder class. This class provides a convenient way to create and manage forms.

5. What’s the best way to work with complex nested forms?

There is no one-size-fits-all answer to this question, as the best way to work with complex nested forms will vary depending on the specific details of the form in question. However, some tips that may be helpful include breaking the form down into smaller manageable pieces, using formArray to manage groups of form controls, and using formGroupName to bind a form control to a specific form group.

6. Is it possible to use template-based validation along with React Forms in Angular? If yes, then how?

Yes, it is possible to use template-based validation along with React Forms in Angular. You can do this by using the ng-template directive. This directive allows you to define a template that will be used to validate the form.

7. Why would you use valueChanges() instead of statusChanges() for a reactive form control?

ValueChanges will give you the current value of the control, while statusChanges will give you the validation status of the control. So, if you are interested in the value of the control, you would use valueChanges.

8. What does the errorStateMatcher property do in an Angular component?

The errorStateMatcher property is used to override the default error matching behavior in Angular. This can be useful if you want to customize how Angular determines whether a form control should be considered invalid.

9. What is the usage of the setValidators() method when used on a form control object in Angular?

The setValidators() method is used to set the validators that will be used to validate the form control object. This is typically used when you want to add new validators to a form control object or when you want to change the existing validators that are being used.

10. What happens if two or more validations return false while processing a single form control?

If two or more validations return false while processing a single form control, then the Angular Reactive Forms module will throw an error.

11. When would you choose one approach over another (template-driven vs reactiive) in angular?

There is no one-size-fits-all answer to this question, as the decision of which approach to take in Angular will depend on the specific requirements of your project. However, some general guidelines can be given.

If you are working on a small project with simple forms, then template-driven forms may be the best option. This approach is also generally easier to learn and use, so it may be a good choice if you are new to Angular.

If you are working on a large project with complex forms, then reactive forms may be a better choice. This approach gives you more control over your form data, and can make handling validation and errors easier.

12. Does Angular provide any built-in support for handling asynchronous data streams coming from different sources?

Yes, Angular provides a built-in support for handling asynchronous data streams coming from different sources. This is done through the use of observables and operators.

13. What’s the role of the FormBuilder class in Angular?

The FormBuilder class is responsible for creating and managing Angular forms. It provides a convenient way to create and manipulate form controls, as well as track and validate form values.

14. What is the purpose of the compose() method in Angular?

The compose() method is used to create a higher-order observable, which is an observable that is composed of multiple other observables. This is often used when working with multiple streams of data that need to be processed together in some way.

15. Can you give me an example of using multiple conditionals within Validators.compose()?

Yes. Validators.compose() accepts an array of Validators, which means you can pass in multiple conditionals. For example, if you wanted to create a Validator that only allowed positive numbers, you could do something like this:

Validators.compose([
Validators.required,
Validators.min(0)
])

16. What is the role of the map() operator in Angular?

The map() operator is used to transform data from one format to another. In the context of Angular, this is often used to convert data from an observable into a format that can be used by the component.

17. What is the role of the mergeMap() operator in Angular?

The mergeMap() operator is used to map each value emitted by the source observable into a new observable, and then flatten the resulting observables into a single observable stream. This is often used when working with asynchronous data sources, such as when making HTTP requests.

18. What’s the importance of RxJS to Angular?

RxJS is a library that Angular uses to help make it easier to work with asynchronous data. This is important because Angular applications often need to work with data that comes from a server or other external source, and this data can arrive at different times. By using RxJS, Angular can more easily handle this asynchronous data and make sure that the application always has the most up-to-date data.

19. What are Observables? Can you explain what they are and give me an example of their use?

Observables are a type of data structure that can be used to represent asynchronous data. This means that they can be used to represent data that changes over time, or data that is being retrieved from a remote source. An example of an observable would be a stream of data that is being emitted from a server.

20. What are the differences between Observables and Promises? Which one should I use in certain situations?

Observables and Promises are both ways of handling asynchronous data. An Observable is a stream of data that can be emitted over time, while a Promise is a single value that is resolved at some point in the future. Promises are often used when you only need to handle a single async event, while Observables are used when you need to handle multiple async events.