Essential Angular Questions for Junior and Mid-Level Job Interviews
Tags: angular, interview, typescript, jobs
Published: 2023-11-11 16:50:02

In the last half-year, I interviewed people to hire Angular Developers for my company. During this time, I learned a lot and noticed common errors made by some applicants.

The purpose of this article is not to provide a one-size-fits-all solution for interviews, but instead to highlight the most common questions and frequently repeated mistakes made by applicants.

Here's a detailed look into the common questions, expected responses, and typical mistakes for junior and mid-level Angular developer interviews.

## **Junior Angular Interview Questions**

These are questions regarding common tasks in Angular, such as hooks, forms, directives, pipes, and guards.

1. **Can you list and explain some key lifecycle hooks in Angular?**
    
    * *Expected Answer:* Expect mentions of `ngOnInit` for component initialization, `ngOnChanges` for detecting changes to input properties, and `ngOnDestroy` for cleanup activities.
        
    * *Common Mistake:* Candidates sometimes fail to correctly sequence the hooks or understand which hook is appropriate for certain activities.
        
2. **What are components in Angular and how are they used?**
    
    * *Expected Answer:* Components are the basic building blocks of Angular applications that control a patch of screen called a view.
        
    * *Common Mistake:* Not understanding the role of the `@Component` decorator or the component lifecycle.
        
3. **Can you explain what services are in Angular and how they are used?**
    
    * *Expected Answer:* Services are designed to encapsulate reusable business logic and functionality that can be injected into components or other services across the application. They are typically used for data fetching, application logic, and data manipulation, among other things. A service with the `@Injectable` decorator allows injecting other dependecies.
        
    * *Common Mistake:* Candidates may often confuse services with components or may not fully grasp the concept of singleton usage in Angular. They may also make the mistake of not using the `providedIn` property correctly, leading to unnecessary imports or incorrect module scope.
        
4. **What is the difference between an Angular module and a JavaScript module?**
    
    * *Expected Answer:* An Angular module is a way to group components, directives, pipes, and services that are related, while JavaScript modules are the standard ES6 modules.
        
    * *Common Mistake:* Not understanding the scope and functionality of an Angular module.
        
5. **What is data binding in Angular?**
    
    * *Expected Answer:* Data binding is the process that allows communication between a component and the DOM, making it easy to define interactive applications without worrying about pushing and pulling data.
        
    * *Common Mistake:* Confusing the syntax for different types of binding, such as property or event binding.
        
6. **How do you create a directive in Angular?**
    
    * *Expected Answer:* Directives are created using the `@Directive` decorator and can manipulate the DOM by changing the appearance or behavior of elements and components.
        
    * *Common Mistake:* Not knowing the difference between structural and attribute directives.
        
7. **What are the two types of forms in Angular and how do they differ?**
    
    * *Expected Answer:* Angular provides two types of forms: Reactive Forms and Template-Driven Forms. Reactive Forms are defined in the component class and use the `FormControl`, `FormGroup`, and `FormArray` classes to track form value changes. On the other hand, Template-Driven Forms are simpler and more linked with the template. They are asynchronous and use directives like `ngModel` within the template itself.
        
    * *Common Mistake:* Might confuse the two types of forms, often misunderstanding the key differences. They might not know which form type is best suited for a given scenario or may incorrectly mix the approaches.
        
8. **What is dependency injection in Angular?**
    
    * *Expected Answer:* Dependency injection is a design pattern in which a class requests dependencies from external sources rather than creating them.
        
    * *Common Mistake:* Not being able to provide examples or articulate the benefits of using dependency injection.
        
9. **What are Angular pipes used for?**
    
    * *Expected Answer:* Pipes are used to transform displayed values within a template.
        
    * *Common Mistake:* Not understanding the difference between pure and impure pipes.
        
10. **Can you explain how routing works in Angular?**
    
    * *Expected Answer:* Routing allows navigation between different views and components based on the URL path.
        
    * *Common Mistake:* Not being aware of route guards or the role of the `RouterModule`.
        
11. **What is TypeScript and why is it used in Angular applications?**
    
    * *Expected Answer:* TypeScript is a superset of JavaScript that compiles to plain JavaScript and provides static typing, classes, and interfaces.
        
    * *Common Mistake:* Believing TypeScript is necessary to run Angular or not understanding its benefits.
        
12. **How would you manage state in an Angular application?**
    
    * *Expected Answer:* The state can be managed using services, component states, or management libraries like NgRx.
        
    * *Common Mistake:* Not being familiar with state management concepts or libraries.
        
13. **What is the purpose of a guard in Angular?**
    
    * *Expected Answer:* Guards are used to control navigation to and from routes in Angular. They can be used to prevent users from accessing certain routes if they don't meet certain conditions like authentication.
        
    * *Common Mistake:* Candidates often confuse guards with simple route checks or fail to mention their use in lazy loading scenarios.
        
14. **How would you validate a form input in Angular?**
    
    * *Expected Answer:* In Angular, you can validate form inputs using built-in validators in both Reactive and Template-Driven Forms. These validators include `required`, `minLength`, `maxLength`, and `pattern`. For Reactive Forms, validators are set as functions within the `FormControl` instances, while in Template-Driven Forms, they are directives placed directly in the HTML template.
        
    * *Common Mistake:* Forget to import the `Validators` class module from `@angular/forms` or apply validators incorrectly in the HTML or component class. They might also not know how to display the error messages related to specific validation errors.
        

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## **Mid-level Angular Interview Questions**

These questions are for developers who have experience with Angular and encounter common problems in Angular applications. We won't be asking about what forms are; instead, we will focus on solutions for typical scenarios in Angular applications, such as performance, RxJS, security, and more.

1. **Explain the concept of lazy loading in Angular.**
    
    * *Expected Answer:* Lazy loading is a technique in Angular that allows for the loading of JavaScript components asynchronously when a specific route is activated.
        
    * *Common Mistake:* Not knowing how to implement lazy loading or the impact it has on performance.
        
2. **How do you optimize an Angular application's performance?**
    
    * *Expected Answer:* Use strategies like trackBy in ngFor, lazy loading strategies, and pure pipes.
        
    * *Common Mistake:* Not being specific or not understanding how these optimizations impact performance.
        
3. **In what scenarios would you use route guards in Angular applications?**
    
    * *Expected Answer:* Route guards are used for authorization checks before a route is activated, to confirm navigation away from a route with unsaved changes, or to pre-load data before route activation.
        
    * *Common Mistake:* May not detail the different types of guards, such as `CanActivate`, `CanDeactivate`, or `Resolve`, and their specific use cases.
        
4. **Discuss the use of RxJS in Angular.**
    
    * *Expected Answer:* RxJS implements reactive programming that can be used to handle asynchronous data calls, event handling, and many other scenarios.
        
    * *Common Mistake:* Having a superficial understanding of observables or not being able to provide practical examples.
        
5. **How do you handle form validation in Angular?**
    
    * *Expected Answer:* Discuss the use of reactive forms with built-in validators or custom validation functions.
        
    * *Common Mistake:* Not mentioning asynchronous validators or the handling of validation messages.
        
6. **Can you explain the concept of a content projection in Angular?**
    
    * *Expected Answer:* Content projection is a way to import HTML content from outside the component into the component's template.
        
    * *Common Mistake:* Not understanding how `<ng-content>` tags work or when to use content projection.
        
7. **Discuss the change detection strategy in Angular.**
    
    * *Expected Answer:* Angular's default change detection strategy is performance-intensive. Changing the strategy to `OnPush` can improve performance by limiting the number of checks.
        
    * *Common Mistake:* Not understanding change detection or when to use different strategies.
        
8. **What are decorators in Angular?**
    
    * *Expected Answer:* Decorators are special kinds of declarations that can attach metadata to classes, methods, properties, or parameters.
        
    * *Common Mistake:* Not being able to differentiate between decorators like `@Component`, `@Injectable`, `@Input`, and `@Output`.
        
9. Can you explain the difference between the `switchMap`, `mergeMap` , `concatMap` and `exhaustMap`operators?
    
    * *Expected Answer:*
        
        * `switchMap`: Cancels the current subscription/request and starts a new one when a new item is emitted by the source observable. It is often used in scenarios like search typeaheads where the latest request is what matters.
            
        * `mergeMap`: Maps each value to an Observable, then flattens all of these inner Observables using `mergeAll`. It’s used when you want to handle all the inner observables concurrently.
            
        * `concatMap`: Maps each value to an Observable, then flattens all of these inner Observables one after the other. This is used when order is important.
            
        * `exhaustMap`: Ignores new values from the source observable while processing a previous one. It’s useful for handling scenarios where you do not want to deal with the new subscription if the previous one is still ongoing, like in the case of button clicks to prevent multiple submissions.
            
    * *Common Mistake:* Mixing up the usage scenarios of these operators or misunderstanding the concurrency and order of execution they offer.
        
10. **How do you ensure security in Angular applications?**
    
    * *Expected Answer:* Use Angular's built-in sanitization to protect against cross-site scripting attacks, use HTTPS, secure the backend API, and implement proper authentication and authorization.
        
    * *Common Mistake:* Overlooking the importance of securing the backend or not knowing about Angular's sanitization features.
        
11. **How would you implement error handling in an RxJS stream?**
    
    * *Expected Answer:* A candidate should talk about the `catchError` operator, which allows interception of errors on the observable stream. They should explain that it can be used to recover from errors by returning a new observable or rethrowing the error. They might also mention the `retry` or `retryWhen` operators for attempting a failed request again.
        
    * *Common Mistake:* Not knowing how to properly handle errors such that the observable sequence is not inadvertently terminated, or failing to understand how to recover from errors and continue the observable sequence.
        
12. **What is the role of NgZone Service in Angular?**
    
    * *Expected Answer:* `NgZone Service` is used for optimizing performance by exiting Angular's zone to prevent Angular from running change detection unnecessarily.
        
    * *Common Mistake:* Not knowing what `NgZone` it is or confusing its purpose with change detection strategies.
        

## Conclusion

These are some of the most common questions in Angular interviews. Of course, the idea is not to remember all the answers, but having a general understanding of each topic can be very helpful.

I recommend consistently practicing to stay sharp. I haven't covered any new Angular topics such as Standalone or Composition API, Signals (coming soon), etc. Most of these questions focus on the fundamentals. If you are well-prepared for these, you will surely pass the interview.

If you'd like to update the article with a question that a company asked you, please feel free to share it in the comments :)