Angular interview questions with answers

ViewChild vs ViewChildren vs ContentChild vs ContentChildren:

• @ViewChild(): Query for a child view element or component projection in the template after Angular has completed its initial compilation and dom manipulations. It allows you to access the ElementRef or the component instance from the parent component, even before the child view is instantiated. It's used for type-safe dynamic view access.

• @ViewChildren(): Query for a query list of child view elements or component projections in the template after Angular has completed its initial compilation and dom manipulations. It provides an Iterable<ElementRef> or Iterable<ComponentInstance>.

• @ContentChild(): Queries for a projected content element or input binding value from parent view to current component’s template, before Angular has completed its initial compilation and dom manipulations. It can be used as both a selector and an input assignment.

• @ContentChildren(): Queries for a query list of projected content elements or input binding values from parent view to the current component’s template, before Angular has completed its initial compilation and dom manipulations.

Importance of Component life cycle:

Understanding the component lifecycle is crucial in Angular as it allows developers to perform necessary initialization and cleanup tasks at specific points in a component's lifecycle. This can include setting up initial values, subscribing to services, making network requests, or cleaning up resources when the component is no longer needed. It also helps in optimizing performance by ensuring that resources are allocated and released at the appropriate times.

Events and sequence of component life cycle:

The events in Angular's component lifecycle follow a specific sequence:

• constructor(): Initialization phase, where you can set up initial values or subscriptions.
• ngOnChanges(): Called when input bindings change.
• ngOnInit(): Called after the constructor, initializing resources that the component will need.
• ngDoCheck(): Detect changes in data-bound expressions and perform additional actions if necessary.
• ngAfterContentInit(): Called after Angular projects additional content into the template, completed after ngOnInit.
• ngAfterContentChecked(): Called after every change detection cycle, used for post-initialization tasks.
• ngAfterViewInit(): Initiated once the component's view and child views have been initialized.
• ngAfterViewChecked(): Called after each change detection cycle, used to check the DOM for changes.
• ngOnDestroy(): Cleanup phase, unsubscribe from observables, remove event listeners etc.

constructor vs ngOnInit():

• The constructor() is a TypeScript lifecycle method that you use to define dependencies and set up initial values or subscriptions. It's called before the first change detection cycle.
• ngOnInit() is specifically designed for components that require initializations after their views have been initialized but before their first change detection cycle. It's a good place to perform initial setup tasks like loading data from a remote service, setting up routing, or subscribing to observables that are only needed once.

How to make HTTP calls using Angular:

Angular provides the HttpClient module for making HTTP requests. You can inject HttpClient into your service and use its methods like get(), post(), put(), delete(), etc., to perform various types of HTTP requests.

import { HttpClient } from '@angular/common/http';

constructor(private http: HttpClient) {}

getUserData(userId: number) {
  return this.http.get(`https://api.example.com/users/${userId}`);
}

Children vs ContentChild/ContentChildren:

@ContentChild() and @ContentChildren() are used to bind input parameters or access content projection within the component's template before Angular has completed its initial compilation and dom manipulations. They are particularly useful when you need to interact with a parent component from a child component through projecting content into it.

ViewChild and ViewChildren, on the other hand, are used for accessing elements or components within the template after Angular has completed its initial compilation and dom manipulations. They are ideal for handling event listeners, manipulating the DOM directly, or interacting with child components as instances rather than just their templates.