Top Angular interview questions and answers

15 Angular questions for interviews#

Angular (also referred to as Angular 2+) has become one of the most widely used frameworks for building complex web applications. Supported by a vast community of developers, Angular is continuously evolving, making it a powerful tool in modern web development. The fifteen Angular questions software developers need to prepare for interviews are as follows:

1. What are the components in Angular?

2. Explain two-way data binding in Angular.

3. What are Angular directives?

4. What is a template?

5. What are the differences between AngularJS and Angular?

6. What is an AOT compilation? What are its benefits?

7. Define life cycle hooks in Angular with examples.

8. What is the difference between a constructor and ngOnInit in Angular?

9. How would you define a single-page application (SPA)?

10. What are annotations?

11. How do we generate a class in Angular using CLI?

12. What is a service in Angular, and how is it used?

13. What are Angular modules?

14. What is the purpose of the ngForOf directive?

15. What is the ngModel directive used for?

1. What are the components in Angular?#

In Angular, a component is a fundamental building block of the user interface. It is a TypeScript class that interacts with the user interface and application logic. Each component is associated with its own view, defined by an HTML template, and logic encapsulated in a TypeScript class.

A component in Angular consists of three main parts:

• Template: Defines the structure and layout of the view (HTML).

• Class: Contains the business logic, properties, and methods (TypeScript).

• Styles: Defines the appearance and styling for the component (CSS or SCSS).

Components are organized hierarchically, where one component can contain other components, making it easier to build reusable and modular code. The component's decorator, @Component, is used to specify metadata such as the selector (custom HTML tag), template, and styles.

2. Explain two-way data binding in Angular#

Two-way data binding in Angular is a feature that enables automatic data synchronization between the view and the model. It allows data to flow in both directions between the component class and the template. This implies that any changes made to the view (user interface) are mirrored instantly in the underlying model (data source), and vice versa.

We can achieve two-way data binding in Angular via a combination of property binding and event binding. The [(ngModel)] directive, imported from the FormsModule, is utilized to bind the value of an input element in the template to a property in the component class. 

3. What are Angular directives?#

Directives are special classes in Angular that add custom behavior to elements or manipulate the DOM. They enable developers to create reusable components that can be utilized across different parts of an application.

In Angular, there are three different kinds of directives:

1. Component directives: Components are a special type of directive with a template. They define new custom elements that can be used as UI components in Angular applications. Every Angular component is essentially a directive with a template.

2. Attribute directives: These directives modify the behavior or appearance of an existing element by changing its attributes. They do not change the structure of the DOM but instead apply visual or behavioral changes. Examples include ngClass and ngStyle.

3. Structural directives: These directives modify the DOM structure by adding, removing, or altering elements. Examples include ngIf, ngFor, and ngSwitch. They use a * prefix when applied in templates, like *ngIf, indicating that they change the layout of the view.

4. What is a template?#

A template in Angular is an HTML-based view that defines how the component’s data is presented to the user. It is a core building block for creating dynamic views that display data and respond to user interactions. Templates contain standard HTML combined with Angular-specific syntax, such as directives and data bindings, which allow you to connect the component’s logic and data to the UI.

Templates use interpolation — a syntax that allows you to embed component properties directly into the HTML. Interpolation is denoted by double curly braces {{}}, and the data or expressions inside them are evaluated and displayed in the view.

For example, if you have a component with a property name set to “Peter,” you can display it in templated code like this:

This will render “Peter” inside an <h1> element when the template is processed by Angular.

5. What are the differences between AngularJS and Angular?#

AngularJS and Angular differ in the following ways: 

6. What is an AOT compilation? What are its benefits?#

AOT compilation is an acronym for “Ahead-of-Time” compilation. It is a process that translates the application's HTML and TypeScript code into optimized JavaScript code during the build phase before the application is loaded in the browser. The result is easy to execute efficient and faster compiled code, enabling an enhanced user experience.

Some of the benefits of AOT compilation in Angular are:

• Improved performance: Since the app is compiled ahead of time, the browser can directly execute the optimized JavaScript code without needing to compile the TypeScript at runtime. This results in faster load times and better overall performance compared to Just-in-Time (JIT) compilation.

• Smaller bundle size: AOT eliminates unnecessary code, metadata, and unused features from the application during the build process, reducing the JavaScript bundle size. This leads to quicker downloads, faster loading, and reduced data usage.

• Enhanced security: With AOT, the compilation happens during the build, meaning there is no need for a runtime compiler in the browser. This reduces the surface area for security vulnerabilities, such as injection attacks, as there’s no dynamic code evaluation at runtime.

• Early error detection: AOT compilation allows for catching template errors and other issues at build time rather than at runtime. This helps developers address issues before deploying the app, resulting in fewer runtime errors and easier debugging.

7. Define life cycle hooks in Angular with examples#

Life cycle hooks are predefined methods in Angular that enable developers to access different stages of a component or a directive’s life cycle. This life cycle begins with the instantiation and rendering of the view and ends with the destruction and removal of the component or directive from the DOM. 

Angular performs change detection throughout the life cycle to update the view and component instance whenever necessary. Developers can utilize life cycle hooks to execute custom code at specific stages, allowing greater control over the functionality and behavior of their components or directives.

Some examples of life cycle hooks are:

• ngOnChanges: This hook is invoked when one or more of the component’s input properties change. It receives an object, which contains the previous and current values of the changed properties.

• ngOnInit: This method is invoked once after the component’s input properties have been initialized. This hook is ideal for performing initialization logic that depends on @Input properties.

• ngDoCheck: This hook is invoked during every change detection cycle, allowing developers to implement custom change detection logic.

• ngAfterContentInit: We invoke this method only after the component's content has been projected into its view.

• ngAfterContentChecked: We invoke this method after the component’s content has been checked for changes.

• ngAfterViewInit: This method is called after we verify that the component’s view has been initialized.

• ngAfterViewChecked: This method is called after the component’s view has been checked for changes.

• ngOnDestroy: This method is invoked before the component’s destruction. It enables you to perform any necessary cleanup logi​​c.

8. What is the difference between a constructor and ngOnInit in Angular?#

A constructor is a special method in Angular that is called whenever a component is created. It’s part of the TypeScript class definition and is used for initializing the class properties and dependencies. ngOnInit, on the other hand, is a life cycle hook called by Angular after initializing the component and its inputs have been set.

The key distinction between the two is that ngOnInit is triggered after Angular initializes the component. ngOnInit is also where the actual business logic (complex initialization) is performed as opposed to the constructor.

Additionally, the constructor is only called once during component creation, while ngOnInit is also called once during the component life cycle—not multiple times. Other life cycle hooks like ngOnChanges handle input changes, not ngOnInit.

9. How would you define a single-page application (SPA)?#

A single-page application (SPA) is a web application that dynamically updates the current web page without requiring a full page reload. We create a SPA in Angular using the Angular framework. It consists of a single HTML page that is dynamically updated with new data from the server as the user engages with the system.

This approach ensures a seamless and smooth user experience akin to a desktop application. The navigation and data exchange between components occur without page reloading, translating into faster and more responsive web applications.

10. What are annotations?#

Annotations are ways of adding metadata to a class or its members using decorators. The decorators are a TypeScript feature—the language used to write Angular applications.

Annotations are specifically used to supply information to the Angular compiler on how to process a class and/or its members. We may use them to specify various things, such as the class’s dependencies, how it should be injected, and how it can be used in templates.

To add annotations to classes, methods, and properties, we use the @ symbol followed by the decorator name. For instance:

In the example above, we applied the @Injectable() decorator to the MySeat class. This decorator instructs Angular that additional dependencies can be injected into this class.

11. How do we generate a class in Angular using CLI?#

We can generate a class in Angular using CLI with the ng generate class command followed by the class name we want to create.

Here’s an example:

The code above will create a new file named myStore.ts in your Angular project's src/app directory (or the current directory, if you’re in a different folder). You can then populate the class with the required properties and methods.  

12. What is a service in Angular, and how is it used?#

A service in Angular is a TypeScript class that provides shared data and functionality among many components, directives, or other services in an application. We can utilize services to encapsulate logic for handling data caching, making API calls, or executing other tasks that require sharing across multiple components.

Angular uses dependency injection (DI), a built-in feature, to inject services into components or other services. This design pattern ensures loose coupling between different parts of the application.

To use a service in Angular, we create a class with the required logic and mark it with the @Injectable() decorator. The service is typically registered with the Angular injector using providedIn: 'root' in the @Injectable() decorator, making it available application-wide. Once registered, the service can be injected into any component or service that needs it. For example:

This example shows how the service is registered globally without needing to  manually add it to a module’s providers array.

13. What are Angular modules?#

Angular modules are a technique to classify and encapsulate related code, like services, components, and directives, into a single cohesive unit. Each Angular application has a single root module that acts as the program’s portal.

Angular modules have several benefits, such as:

• Encapsulation: Modules help you group together related code and avoid name collisions with other modules’ code.

• Lazy loading: Your application’s performance is enhanced when modules are lazy loaded-that is, only when required.

• Reusability: We can reuse modules across different applications, making it easier to share code between projects.

• Dependency management: Modules enhance the dependency management of your application by declaring their dependencies on other modules.

14. What is the purpose of the ngForOf directive?#

The shorthand form ngFor is commonly used in Angular templates, while the full directive name is ngForOf. It is used to iterate over a collection of data (such as an array) and instantiate a view for each item in the collection. The ngFor directive is typically used to display lists of items, such as products, tasks, or comments.

For example;

In the example above, the ngFor directive iterates over the lists array and generates an <li> element for each item in the array. The local variable list represents each item in the iteration, giving access to its properties (e.g., title and content).

15. What is the ngModel directive used for?#

The ngModel directive is used for two-way data binding in Angular, linking the value of HTML form controls like input, select, and textarea to a property in the component’s data model. It is a part of the FormsModule and allows the model to be updated whenever the form input changes, and vice versa.

For example:

In this example, the username property in the component is bound to the input field, so any changes to the field are automatically reflected in the username variable, and changes to username are updated in the input field.

More Angular questions to practice#

To further enhance your understanding of Angular, here are additional practice questions that cover a wide range of concepts, helping you prepare more effectively for interviews.

• Explain the difference between a component and a directive in Angular.

• How does the MVVM architecture work in Angular?

• How do a feature module and a shared module differ?

• What is the main purpose of the Angular CLI? 

• What is an Angular route guard? How does an application safeguard its routes using it?

• What are dependency injections in Angular? How are they used?

• How do we use an async pipe in Angular? 

• What does an Angular template reference variable mean? How does it work to access template elements?

• How can you create a simple animation module in Angular?

• What is the main difference between a pure and an impure pipe in Angular? When should you use one over the other?

Next step in your Angular interview prep#

We hope you enjoyed brushing up on your Angular skills! Angular is a robust framework with a lot to offer for developers and is here to stay. While it can be daunting to learn and understand its core concepts, familiarizing yourself with common interview questions and answers and practically applying these concepts can help you crack any interview and become a proficient Angular developer. 

Remember, greatness is achieved with practice, which is key when it comes to mastering Angular. Don’t be afraid to seek help from the extensive developer community whenever you need it. Keep exploring, learning, and building amazing applications with Angular!

Happy learning!

Ready to dive deeper? Explore our interactive skills path on Educative to build scalable Angular applications, from beginner concepts to advanced techniques.