Angular Interview Questions and Answers
- What are Angular Guards? Explain their types and when to use them.
Angular Guards are mechanisms in Angular applications used to control access to certain parts of the application. They are used to protect routes, either by allowing or denying access based on certain conditions. Guards can be implemented as services that Angular's router calls when navigating to a route. There are several types of guards in Angular:
CanActivate: This guard determines if a route can be activated or not. It's commonly used to check if a user is authenticated before allowing access to a particular route. If the CanActivate guard returns true, navigation proceeds, otherwise, it's blocked.
CanActivateChild: Similar to CanActivate, but specifically for child routes. It's used to guard the child routes of a component.
CanDeactivate: This guard determines if a route can be deactivated or not. It's useful for confirming with users before leaving a page with unsaved changes.
CanLoad: This guard is used to prevent asynchronous routing by checking if a module can be loaded lazily or not. It's typically used to prevent loading feature modules if certain conditions are not met, like if a user is not authenticated.
Resolve: This guard performs route data retrieval before route activation. It's useful for resolving data dependencies before the component is activated.
Guards are implemented as Angular services and can be added to route configurations in the Angular router. They are used in conjunction with route definitions to control access to routes based on specific conditions.
2. What is Angular Universal? Explain its purpose.
Angular Universal is a technology provided by the Angular framework that enables server-side rendering (SSR) of Angular applications. The purpose of Angular Universal is to improve the performance and user experience of Angular applications by rendering them on the server and sending fully rendered HTML to the client, rather than relying solely on client-side rendering (CSR).
The key purposes of Angular Universal include:
Improved performance: Server-side rendering reduces the time to first contentful paint (FCP) and improves perceived performance for users. By rendering the initial HTML on the server and sending it to the client, users can see the content sooner, especially on slower devices or networks.
Search engine optimization (SEO): Search engines typically have difficulty crawling and indexing client-rendered JavaScript applications. By providing pre-rendered HTML to web crawlers, Angular Universal improves SEO and ensures that search engines can properly index the content of Angular applications.
Better social sharing: When users share links to Angular application pages on social media platforms, those platforms often rely on the initial HTML content to generate previews. Angular Universal ensures that the shared links display the correct content by providing pre-rendered HTML.
Enhanced user experience: Server-side rendering can help provide a consistent user experience across different devices and browsers, as users receive fully rendered content regardless of their device's capabilities or browser support for JavaScript.
Compatibility with legacy systems: Server-side rendering enables Angular applications to integrate more seamlessly with existing server-side technologies, such as content management systems (CMS) or legacy server frameworks.
Overall, Angular Universal enhances the performance, SEO, and user experience of Angular applications by rendering them on the server and sending fully rendered HTML to the client, complementing the traditional client-side rendering approach.
3. Differentiate between ngOnChanges() and ngOnInit().
ngOnChanges() and ngOnInit() are lifecycle hooks provided by Angular that serve different purposes. Here's how they differ:
ngOnChanges():
This lifecycle hook is invoked every time there is a change in the input properties of the component.
It receives a SimpleChanges object as an argument, which contains the previous and current values of the input properties that have changed.
It is commonly used to perform actions based on changes to input properties, such as updating component state or triggering side effects.
Example:
import { Component, OnChanges, SimpleChanges } from '@angular/core';
@Component({
selector: 'app-example',
template: '...'
})
export class ExampleComponent implements OnChanges {
ngOnChanges(changes: SimpleChanges): void {
// Perform actions based on changes to input properties
if (changes.inputProperty) {
// Handle changes to inputProperty
}
}
}
ngOnInit():
This lifecycle hook is invoked once, after Angular has initialized the component's properties.
It is commonly used to perform initialization tasks for the component, such as fetching initial data from a server, initializing component state, or setting up subscriptions.
It's important to note that ngOnInit() is called after the first ngOnChanges().
Example:
import { Component, OnInit } from '@angular/core';
@Component({
selector: 'app-example',
template: '...'
})
export class ExampleComponent implements OnInit {
ngOnInit(): void {
// Perform initialization tasks
// Fetch initial data, set up component state, etc.
}
}
In summary, ngOnChanges() is invoked whenever there is a change in the input properties of the component, while ngOnInit() is invoked once, after the component's properties are initialized, and it is commonly used for initialization tasks.
}