Question 1

What is the difference between ngOnInit and constructor in Angular?

Accepted Answer

The constructor is used for initializing class members and injecting dependencies. ngOnInit is a lifecycle hook called by Angular after the component is initialized. It's best to perform component-specific logic like data fetching inside ngOnInit rather than in the constructor.

Question 2

How does Angular’s change detection mechanism work?

Accepted Answer

Angular uses a unidirectional tree-based change detection mechanism. When a change is triggered, Angular runs change detection from the top of the component tree to the bottom, checking for changes in component data and updating the DOM accordingly.

Question 3

What is the difference between Subject and BehaviorSubject in RxJS?

Accepted Answer

A Subject does not hold a value and only emits to subscribers after they subscribe. A BehaviorSubject holds a value and emits the current value immediately to new subscribers. This makes BehaviorSubject useful for storing and broadcasting state.

Question 4

How does dependency injection work in Angular?

Accepted Answer

Angular’s DI framework allows components to declare dependencies that Angular will automatically resolve and inject. Services can be provided at component or module level, with different scopes like singleton or multi-instance based on the provider configuration.

Question 5

What is the role of NgModules in Angular architecture?

Accepted Answer

NgModules help organize an Angular application into cohesive blocks of functionality. They group related components, directives, services, and pipes. Angular apps start with a root module and can have feature modules for better scalability and lazy loading.

Question 6

What is the purpose of ngOnChanges lifecycle hook?

Accepted Answer

ngOnChanges is called when an input-bound property changes. It receives a SimpleChanges object containing the previous and current values of the changed inputs, allowing you to respond to changes.

Question 7

What is the difference between ngAfterContentInit and ngAfterViewInit?

Accepted Answer

ngAfterContentInit is called after content projection (ng-content) is initialized, while ngAfterViewInit is called after the component's view and child views are initialized.

Question 8

What is the purpose of ngOnDestroy?

Accepted Answer

ngOnDestroy is called just before Angular destroys the component or directive. It is used for cleanup, such as unsubscribing from observables and detaching event handlers to avoid memory leaks.

Question 9

Can you explain the order of lifecycle hooks?

Accepted Answer

The order is: constructor → ngOnChanges → ngOnInit → ngDoCheck → ngAfterContentInit → ngAfterContentChecked → ngAfterViewInit → ngAfterViewChecked → ngOnDestroy.

Question 10

What is ChangeDetectionStrategy.OnPush and when should you use it?

Accepted Answer

OnPush tells Angular to run change detection only when input references change, events originate from the component or its children, or you manually trigger change detection. It improves performance by reducing unnecessary checks.

Angular Advanced Interview Questions & Answers

What is the difference between ngOnInit and constructor in Angular?

How does Angular’s change detection mechanism work?

What is the difference between Subject and BehaviorSubject in RxJS?

How does dependency injection work in Angular?

What is the role of NgModules in Angular architecture?

What is the purpose of ngOnChanges lifecycle hook?

What is the difference between ngAfterContentInit and ngAfterViewInit?

What is the purpose of ngOnDestroy?

Can you explain the order of lifecycle hooks?

What is ChangeDetectionStrategy.OnPush and when should you use it?

How do you manually trigger change detection in Angular?

What is the role of NgZone in change detection?

How does Angular handle change detection for arrays and objects?

What is the difference between ChangeDetectorRef.detectChanges() and ApplicationRef.tick()?

What is the difference between switchMap, mergeMap, and concatMap?

What is the purpose of the shareReplay operator?

How do you handle errors in RxJS observables?

What is the AsyncPipe and why use it?

How do you implement a polling mechanism with RxJS?

Explain the concept of a cold vs hot observable.

What are the different provider scopes in Angular?

What is the difference between @Injectable() and @Inject()?

What is the difference between a component and a directive?

Explain the concept of lazy loading in Angular.

What is the difference between forRoot and forChild in NgModules?

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