Angular is one of the most popular JavaScript single-page application frameworks. It is also quite vast and difficult to navigate. Even more difficult is to get answers to the interview questions on Angular as it is very confusing to know which parts of the Angular framework you should familiarize yourself with. Keeping that in mind, we have come up with a compilation of Angular interview questions and answers that are asked in these interviews most frequently. Being able to answer these questions will give you a great chance of acing your Front End Angular Developer interview.

Most-Asked Angular Interview Questions and Answers

Q1. What is Angular?
Q2. What are the technologies used in Angular?
Q3. Why were client-side frameworks like Angular introduced?
Q4. What are some advantages of using Angular?
Q5. What are some disadvantages of using Angular?
Q6. What is an AOT compilation in Angular?
Q7. What are the advantages of AOT?
Q8. What are the components in Angular?
Q9. What are modules in Angular?
Q10. What are services in Angular?

These Interview questions on Angular are divided into two sections which are better organized for job interview preparation. These sections will guide you through the basic and advanced Angular Interview questions.

1. Angular Basic Interview Questions

2. Angular Advanced Interview Questionss

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Top 30 Angular Interview Questions and Answers 2021

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Angular Basic Interview Questions

Let’s start with the basic Angular interview questions and answers for freshers.

1. What is Angular?

Angular is an open-source web application development framework created by Google. It is used to build frontend, single-page applications that run on JavaScript. It is a full-fledged framework, i.e., it takes care of many aspects of frontend web applications such as HTTP requests, routing, layout, forms, reactivity, validation, etc.

2. What are the technologies used in Angular?

Angular is a modern frontend JavaScript framework developed by Google. Angular itself makes use of several technologies for several reasons to accomplish certain tasks easily as well as to allow developers to have a better experience while developing applications with it. Angular uses TypeScript, which is a superscript of JavaScript. So, any valid JavaScript is a valid TypeScript. However, TypeScript allows us to write JavaScript as a strongly typed language, and we can define our own types as well, which makes catching bugs much easier. It also uses RxJS, which allows developers to better deal with asynchronous operations.

3. Why were client-side frameworks like Angular introduced?

Before JavaScript-based client-side frameworks, the way dynamic websites worked was by taking a template that is nothing but HTML code with spaces left empty for feeding data and content into those templates. This data was usually fetched from a database. After combining the template and data, we would serve the generated HTML content back to the user. As you can see, it was a bit complicated, and in some cases, it took a lot of processing.

 To overcome these issues, people came up with another approach in which they send the necessary data to render a page from their web servers to the web browsers and let JavaScript combine this data with a predefined template. Since now, even mobile phones are powerful enough to do this kind of processing, the servers can now just send the data to a client over the internet in a recognizable format, i.e., JSON, XML, etc. This drastically reduces the processing done on the servers and improves performance.

4. What are some advantages of using Angular?

Using Angular has several advantages, which are listed below:

  • Angular is built using TypeScript, which allows developers to write strongly typed code that will get transpiled into JavaScript. The benefits of strongly typed code are that it is easy to read, maintainable, and less prone to errors. Also, it provides better tooling with type hints and code completion.
  • Angular allows us to separate our code into modules, which can be used to wrap functionalities related to a specific task such as HTTP communication, data validation, routing, etc.
  • Angular has a large ecosystem of tools, libraries, frameworks, plugins, etc. that make the whole development experience much faster and enjoyable. These tools and libraries include Angular CLI, RxJS, NgRx, etc.

5. What are some disadvantages of using Angular?

Although Angular provides quite a lot of benefits, there are some disadvantages of using it as well. They are as follows:

  • Getting good SEO results on an Angular application can be a bit difficult and may need a bit of configuration.
  • Angular has a lot of features packed into it, so getting to know each of them and learning how to use them effectively together can be a little difficult.
  • Angular can add quite a lot of weight to your JavaScript bundle, so using it for smaller projects can be very inefficient and may significantly increase the load size.

6. What is an AOT compilation in Angular?

The AOT (ahead-of-time) compiler in Angular converts Angular HTML and TypeScript code into JavaScript code during the build phase, which makes the rendering process much faster. This compilation process is needed since Angular uses TypeScript and HTML code. The compiler converts the code into JavaScript, which can then be effectively used by the browser that runs our application.

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7. What are the advantages of AOT?

AOT compilation has several advantages as mentioned below:

  • Fast rendering: Since, after compilation, the browser would download a pre-compiled version of our application, it can render the application immediately without compiling the app.
  • Less asynchronous requests: It takes external HTML templates and CSS style sheets and inlines them within the application JavaScript, which reduces the number of separate Ajax requests.
  • Smaller download size: The compiler will minify the code for us so that the download size is less.
  • Template error detection: During the compilation phase, any issues in the templates will be detected and reported by the compiler so that they can be corrected before production.

8. What are the components in Angular?

Components are the basic building block of the user interface in Angular. A component consists of HTML, CSS, and JavaScript for a specific portion of a user interface. We can think of these as a custom HTML element that only Angular can understand. These components are isolated, i.e., styles and code from one component do not affect other components as they get namespaced by the compiler. These components are then pieced together by the Angular framework to build the user interface for the browser to render.

9. What are modules in Angular?

A module is a logical boundary of our application. It is used to encapsulate code dealing with a specific aspect of the application, such as routing, HTTP, validation, etc. The main reason why modules are used is to enhance application composability. For example, if we wish to implement validation logic using different libraries, then for the one we have already implemented, we can create a new validation module and replace the current one with the new one, and our application would work just the same. In Angular, we create a module using the NgModule decorator.

10. What are services in Angular?

A service in Angular is a term that covers broad categories of functionalities. A service is any value, function, or feature that an app needs. A service is typically used to accomplish a very narrow purpose such as HTTP communication, sending data to a cloud service, decoding some text, validating data, etc. A service does one thing and does it well. It is different from a component as it is not concerned with HTML or any other kind of presentation logic. Normally, a component uses multiple services to accomplish multiple tasks.

11. What are lifecycle hooks in Angular?

When building an Angular app, there will be times when we need to execute some code at some specific event—such as when a component is initialized or displayed on the screen or when the component is being removed from the screen. This is what lifecycle hooks are used for. For example, if we have some event listener attached to an HTML element in a component, such as a button click or form submission, we can remove that event listener before removing the component from the screen, just like we can fetch some data and display it on the screen in a component after the component is loaded on the screen. To use a lifecycle hook, we can override some methods on a component, such as ngOnInit or ngAfterViewInit. These methods, if available on a component, will be called by Angular automatically. This is why these are called lifecycle hooks.

12. What is a two-way data binding?

Two-way data binding is done in Angular to ensure that the data model is automatically synchronized in the view. For example, when a user updates some data in a model and that model is being displayed in multiple places in a component, that update should be reflected in all the places.

Two-way data binding has been supported in Angular for a long time. Although, it is something that should be used with careful consideration as it could lead to poor application performance or performance degradation as time goes on. It is called two-way data binding because we can change some data that is in the component model from the view that is HTML, and that change can also propagate to all other places in the view where it is displayed.

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13. What are pipes in Angular?

When we are trying to output some dynamic data in our templates, we may sometimes need to manipulate or transform the data before it is put into our templates. Though there are several ways of doing that, in Angular, using pipes is the most preferred way. A pipe is just a simple function, which we can use with expressions in our templates. 

Pipes are extremely useful as we can use them throughout our application after declaring them just once and registering them with the Angular framework. Some of the most common built-in pipes in Angular are UpperCasePipe, LowerCasePipe, CurrencyPipe, etc.

14. What are observables in Angular?

An observable is a declarative way using which we can perform asynchronous tasks. Observables can be thought of as streams of data flowing from a publisher to a subscriber. They are similar to promises as they both deal with handling asynchronous requests. However, observables are considered to be a better alternative to promises as the former comes with a lot of operators that allow developers to better deal with asynchronous requests, especially if there is more than one request at a time.

Observables are preferred by many developers as they allow them to perform multiple operations such as combining two observables, mapping an observable into another observable, and even piping multiple operations through an observable to manipulate its data.

15. How are observables different from promises?

Although both promises and observables are used to handle asynchronous requests in JavaScript, they work in very different ways. Promises can only handle a single event at a time, while observables can handle a sequence of asynchronous events over a period of time. Observables also provide us with a wide variety of operators that allow us to transform data flowing through these observables with ease. 

A promise is just a way to wrap asynchronous operations so that they can be easily used, while an observable is a way to turn asynchronous operations into a stream of data that flows from a publisher to a subscriber through a well-defined path with multiple operations transforming the data along the way.

16. What is RxJS?

RxJS is a library, and the term stands for Reactive Extensions for JavaScript. It is used so that we can use observables in our JavaScript project, which enables us to perform reactive programming. RxJS is used in many popular frameworks such as Angular because it allows us to compose our asynchronous operations or callback-based code into a series of operations performed on a stream of data that emits values from a publisher to a subscriber. Other languages such as Java, Python, etc. also have libraries that allow them to write reactive code using observables.

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H2: Angular Advanced Interview Questions

Let’s start with the advanced Angular interview questions and answers for experienced users.

17. What does subscribing mean in RxJS?

In RxJS, when using observables, we need to subscribe to an observable to use the data that flows through that observable. This data is generated from a publisher and is consumed by a subscriber. When we subscribe to an observable, we pass in a function for the data and another function for errors so that, in case there is some error, we can show some message or process the message in some way.

18. What is Angular Router?

Routing in a single-page frontend application is the task of responding to the changes in the URL made by adding and removing content from the application. This is a complicated task as we first need to intercept a request that changes the browser’s URL as we do not wish for the browser to reload. Then, we need to determine which content to remove and which content to add, and finally, we have to change the browser’s URL as well to show the user the current page they are on. 

As we can see, this can be very difficult to implement, especially in multiple applications. That is why Angular comes with a full routing solution for a single-page application. In this, we can define routes with matching components and let Angular handle the routing process.

19. What is HttpClient, and what are its benefits?

HttpClient is an Angular module used for communicating with a backend service via the HTTP protocol. Usually, in frontend applications, for sending requests, we use the fetch API. However, the fetch API uses promises. Promises are useful, but they do not offer the rich functionalities that observables offer. This is why we use HttpClient in Angular as it returns the data as an observable, which we can subscribe to, unsubscribe to, and perform several operations on using operators. Observables can be converted to promises, and an observable can be created from a promise as well.

20. What is multicasting in Angular?

In Angular, when we are using the HttpClient module to communicate with a backend service and fetch some data, after fetching the data, we can broadcast it to multiple subscribers, all in one execution. This task of responding with data to multiple subscribers is called multicasting. It is specifically useful when we have multiple parts of our applications waiting for some data. To use multicasting, we need to use an RxJS subject. As observables are unicast, they do not allow multiple subscribers. However, subjects do allow multiple subscribers and are multicast. 

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21. What is a directive in Angular?

A directive in Angular is used to extend the syntax and capabilities of a normal HTML view. Angular directives have special meaning and are understood by the Angular compiler. When Angular begins compiling the TypeScript, CSS, and HTML files into a single JavaScript file, it scans through the entire code and looks for a directive that has been registered. In case it finds a match, then the compiler changes the HTML view accordingly. 

Angular is shipped with many directives. However, we can build our directives and let Angular know what they do so that the compiler knows about them and uses them during the compilation step.

22. Explain different kinds of Angular directives.

There are three kinds of directives in Angular. Let’s discuss them:

  • Components: A component is simply a directive with a template. It is used to define a single piece of the user interface using TypeScript code, CSS styles, and the HTML template. When we define a component, we use the component decorated with the @ symbol and pass in an object with a selector attribute. The selector attribute gives the Angular compiler the HTML tag that the component is associated with so that, now, when it encounters this tag in HTML, it knows to replace it with the component template.
  • Structural: Structural directives are used to change the structure of a view. For example, if we wish to show or hide some data based on some property, we can do so by using the ngIf directive, or if we wish to add a list of data in the markup, we can use *ngFor, and so on. These directives are called structural directives because they change the structure of the template.
  • Attribute: Attribute directives change the appearance or behavior of an element, component, or another directive. They are used as the attributes of elements. Directives such as ngClass and ngStyle are attribute directives.

23. What are the different types of compilers used in Angular?

In Angular, we use two different kinds of compilers:

  • Just-in-time (JIT) compiler
  • Ahead-of-time (AOT) compiler

Both these compilers are useful but for quite different purposes. The JIT compiler is used to compile TypeScript to JavaScript as our browsers cannot understand TypeScript but only JavaScript. This compilation step is done in a development environment, i.e., when less time is needed to be spent on compilation and more in development to quickly iterate over features. The JIT compiler is used when we use ng serve or ng build command to serve the app locally or create an uncompressed build of the entire codebase.

On the other hand, the AOT compiler is used to create a minified production build of the entire codebase, which can be used in production. To use the AOT compiler, we have to use the ng build command with the –prod blog: ng build –prod. This instructs the Angular CLI to create an optimized production build of the codebase. This takes a bit more time because several optimizations such as minification can take time, but for production builds, this time can be spared.

Read the differences Angularjs, Angular 2 and Angular 4 in our comparative blog on Angularjs vs Angular 2 vs Angular 4!

24. What is the purpose of the common module in Angular?

In Angular, the common module that is available in the package @angualr/common is a module that encapsulates all the commonly needed features of Angular, such as services, pipes, directives, etc. It contains some sub-modules as well such as the HttpClientModule, which is available in the @angular/common/http package. Because of the modular nature of Angular, its functionalities are stored in small self-contained modules, which can be imported and included in our projects if we need these functionalities.

25. What are the differences between AngularJS and Angular?

AngularJS is the previous version of Angular, which is a complete rewrite, i.e., there are several differences between the two that we can highlight.

  • Architecture: AngularJS supports the MVC architecture in which the model contains the business logic; the view shows the information fetched from the models, and the controller manages interactions between the view and the model by fetching data from the model and passing it to the view. On the other hand, in Angular, we have a component-based architecture where instead of having separate pieces for logic, presentation, etc., we now have a single self-contained piece of the user interface that can be used in isolation or included in a big project.
  • Language: In AngularJS, we could only use JavaScript. However, in Angular, we can use both TypeScript and JavaScript.
  • Mobile support: In AngularJS, we do not get mobile browser support out of the box, but in Angular, we do get mobile support for all popular mobile browsers.

26. What are the differences between Angular expressions and JavaScript expressions?

Angular expressions and JavaScript expressions are quite different from each other as, in Angular, we are allowed to write JavaScript in HTML, which we cannot do in plain JavaScript. Also, all expressions in Angular are scoped locally. But, in JavaScript, these expressions are scoped against the global window object. These differences, however, are reconciled when the Angular compiler takes the Angular code we have written and converts it into plain JavaScript, which can then be understood and used by a web browser.

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27. What is server-side rendering in Angular?

In a normal Angular application, the browser executes our application, and JavaScript handles all the user interactions. However, because of this, sometimes, if we have a large application with a big bundle size, our page’s load speed is slowed down quite a bit as it needs to download all the files, parse JavaScript, and then execute it. To overcome this slowness, we can use server-side rendering, which allows us to send a fully rendered page from the server that the browser can display and then let the JavaScript code take over any subsequent interactions from the user.

28. What is Angular Universal?

Angular Universal is a package for enabling server-side rendering in Angular applications. We can easily make our application ready for server-side rendering using the Angular CLI. To do this, we need to type the following command:

ng add @nguniversal/express-engine

This allows our Angular application to work well with an ExpressJS web server that compiles HTML pages with Angular Universal based on client requests. This also creates the server-side app module, app.server.module.ts, in our application directory.

29. What is the difference between interpolated content and the content assigned to the innerHTML property of a DOM element?

Angular interpolation happens when in our template we type some JavaScript expression inside double curly braces ‘{{ someExpression() }}’. This is used to add dynamic content to a web page. However, we can do the same by assigning some dynamic content to the innerHTML property of a DOM element. The difference between the two is that, in Angular, the compiler always escapes the interpolated content, i.e., HTML is not interpreted, and the browser displays the code as it is with brackets and symbols, rather than displaying the output of the interpreted HTML. However, in innerHTML, if the content is HTML, then it is interpreted as the HTML code.

30. What are HttpInterceptors in Angular?

HttpInterceptors are part of the @angular/common/http module and are used to inspect and transform HTTP requests and HTTP responses as well. These interceptors are created to perform checks on a request, manipulate the response, and perform cross-cutting concerns, such as logging requests, authenticating a user using a request, using gzip to compress the response, etc.

If you have any doubts or queries related to Angular, get them clarified from the AngularJs experts on our Web Development Community! 

Hopefully, these Angular interview questions in 2021 have helped you get a better grasp of Angular as a framework, as well as its various features and capabilities. These frequently asked questions have been created to give you a better understanding of the kinds of questions asked in interviews, so they will help you in understanding Angular interviews and Angular as a framework. These Angular interview questions with their answers might have whetted your appetite for learning more about the framework. They will surely help you to ace your next job interview.

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