15 common Angular interview questions and answers

Angular is a popular open-source application-design framework and development platform. Google initially released it in 2010 as AngularJS, but it underwent a major overhaul in 2016 and was re-released as Angular (also known as Angular 2+). Subsequently, it has become one of the most popular frameworks for building complex web applications, supported by a vast community of developers continuously contributing to its development. It comes with various capabilities, including a powerful command-line interface feature, dependency injection, two-way data binding, and component-based architecture, making it a robust tool for developing highly responsive and dynamic applications.

A major advantage of Angular is that it enables developers to build cross-platform applications, including web, desktop, and mobile apps, using a single codebase. This encourages a write once and deploy anywhere code approach across multiple platforms — saving time and effort.

Given Angular's widespread use, there is a high demand for developers with expertise in this framework. Therefore, it is important to prepare well for Angular-related interviews. This blog is designed to assist you with this task by answering commonly asked Angular questions. We have gathered 15 of the most popular questions and answers covering topics ranging from component architecture to data binding.

So, let's dive in and level up your Angular knowledge!

What is Angular?

Angular is an open-source development platform built on TypeScript. It offers a component-based framework and various well-integrated libraries to help develop scalable web applications.

The platform also has various developer tools that make creating, testing, refactoring, and updating code easy.

One of Angular's key advantages is its scalability–it allows for developing both small single-developer projects and large enterprise-level applications. Additionally, it makes updating apps simple and straightforward.

It is also highly diverse - with over 2 million developers, library authors, and content producers contributing to the growth and development of the Angular ecosystem. 

2. What are Angular directives?

Directives are classes in Angular that add custom behavior to elements and components. 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: These are directives that define new custom elements, which can be used as UI components in an Angular application.

2. Attribute directives: These are directives that modify the behavior or appearance of an existing element by changing the appearance of attributes.

3. Structural directives: These are directives that change the template's structure by adding, removing, or modifying DOM elements.

3. What is a template?

A template in Angular is an HTML form that renders a component. It is a basic building block for creating views that display data to the user. Templates contain HTML files with Angular-specific markups, such as directives and bindings, allowing you to bind the view to the component's data and logic.

Templates use interpolation — a syntax which allows you to embed component data directly into the HTML. Interpolation is denoted by the double curly braces {{}}, and the data to be displayed is placed inside them. 

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

4. What are the differences between AngularJS and Angular?

AngularJS and Angular differ in the following ways: 

5. 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. 

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: AOT-compiled code executes faster than just-in-time (JIT) compiled code, resulting in quicker load times and improved performance.

• Smaller bundle size: By eliminating superfluous code and metadata, AOT compilation can minimize the size of the JavaScript bundle for the application, resulting in quicker downloads and less data usage.

• Enhanced security: Since the AOT-compiled code has already been translated into JavaScript, you won't need a runtime compiler. This minimizes the risk of security vulnerabilities like injection attacks and boosts overall security.

• Early error detection: AOT compilation helps detect errors early at build time, allowing developers to catch errors before deploying the application. This makes debugging easier by reducing the number of errors caught at runtime. 

7 . Define lifecycle hooks in Angular with examples

Lifecycle hooks are predefined methods in Angular that enable developers to access different stages of a component or a directive's lifecycle. This lifecycle 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 lifecycle to update the view and component instance whenever necessary. Developers can utilize lifecycle hooks to execute custom code at specific stages, allowing greater control over the functionality and behavior of their components or directives.

Some examples of lifecycle hooks are:

• ngOnChanges: When one or more input properties of a component change, we invoke this method. It receives an object containing the input properties' prior and present values.

• ngOnInit: This method is invoked after component initialization and when input properties have been set. It is an ideal place to perform any initialization logic that depends on the input properties.

• ngDoCheck: We call this method during every change detection cycle of the component. It enables you to 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 cleanup logic necessary.

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

A constructor is a special method in Angular that gets 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 lifecycle 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 may be called multiple times based on the component's input change. 

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 and it consists of a single HTML page, which 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 occurs 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. 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 is used to share 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.

Services are injected into components via a dependency injection — which is a design pattern that allows loose coupling between components. 

To use a service in Angular, you must first define it by creating a class that implements the service's functionality. Then, you have to register the service with the Angular injector by adding it to the provider's array of the module that will utilize it. After service registration, you can inject it into any component or other service that needs it. For example;

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 for the ngForOf directive in Angular. It is used to iterate over a collection of data and create a template for each item in the collection. The ngFor directive is mainly utilized to display a list of items, such as a list of products, shopping items, or comments.

For example;

The ngFor directive in the above example loops through the lists array and generates an <li> element for every list. You can typically access the properties of the current list using the list local variable.

15. What is the ngModel directive used for?

The ngModel directive links the value of HTML controls like input, select, and text-area, to the data from an application. It is a part of the FormsModule and can be used independently or as a component of a larger form. It also has the ability to take a domain model as an input parameter.

More Angular questions to practice

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

17. How does the MVVM architecture work in Angular?

18. How do a feature module and a shared module differ?

19. What is the main purpose of the Angular CLI? 

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

21. What are dependency injections in Angular? How are they used?

22. How do we use an async pipe in Angular? 

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

24. How can you create a simple animation module in Angular?

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

Take the 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 and seek help from the extensive developer community whenever you need it. Keep exploring, learning, and building amazing applications with Angular!

Happy learning!