27 Angular 9 Interview Questions (with Ivy UPDATE)

Observables are declarative which provide support for passing messages between publishers and subscribers in your application.

They are mainly used for event handling, asynchronous programming, and handling multiple values. In this case, you define a function for publishing values, but it is not executed until a consumer subscribes to it. The subscribed consumer then receives notifications until the function completes, or until they unsubscribe.

@HostBinding lets you set properties on the element or component that hosts the directive.

The code applies the css class valid to whatever is using this directive conditionally based on the value of isValid.

Promises:

• return a single value
• not cancellable
• more readable code with try/catch and async/await

Observables:

• work with multiple values over time
• cancellable
• support map, filter, reduce and similar operators
• use Reactive Extensions (RxJS)
• an array whose items arrive asynchronously over time

• The Constructor is a default method of the class that is executed when the class is instantiated and ensures proper initialization of fields in the class and its subclasses.
• ngOnInit is a life cycle hook called by Angular to indicate that Angular is done creating the component. We have to import OnInit in order to use like this (actually implementing OnInit is not mandatory but considered good practice).

Mostly we use ngOnInit for all the initialization/declaration and avoid stuff to work in the constructor. The constructor should only be used to initialize class members but shouldn't do actual "work".

Angular Universal is a server-side rendering module for Angular applications in various scenarios. This is a community driven project and available under@angular/platform-server package. Recently Angular Universal is integrated with Angular CLI.

Redux is a way to manage application state and improve maintainability of asynchronicity in your application by providing a single source of truth for the application state, and a unidirectional flow of data change in the application. ngrx/store is one implementation of Redux principles.

• Virtual DOM creates a whole tree from scratch every time you rerender.
• Incremental DOM, on the other hand, doesn’t need any memory to rerender the view if it doesn’t change the DOM. We only have to allocate the memory when the DOM nodes are added or removed. And the size of the allocation is proportional to the size of the DOM change.