Angular Interview Questions

Dependency Injection (DI) is a design pattern used in Angular to create and manage the dependencies of various components, services, and other classes. It allows a class to receive its dependencies from an external source rather than creating them itself. This approach promotes modularity, maintainability, and testability in Angular applications.

Key Concepts of Dependency Injection in Angular:

1. Dependency: An object or service that a class needs to perform its function.
2. Injector: A service locator responsible for instantiating dependencies and injecting them where needed.
3. Provider: Configuration metadata that tells the Angular injector how to create the dependency.

How Dependency Injection is Implemented in Angular:

1. Providers:

   • Definition: Providers define how to create an instance of a dependency.
   • Registration: Providers are registered in an Angular module (typically in the providers array of @NgModule), component, or service.
   • Example: Registering a service provider.

     @NgModule({
       providers: [MyService]
     })
     export class AppModule { }

2. Injectable Decorator:

   • Definition: The @Injectable decorator marks a class as available to the Angular DI system.
   • Purpose: It informs Angular that the class can be used as a dependency and should be managed by the injector.
   • Example: Marking a service as injectable.

     import { Injectable } from '@angular/core';
     
     @Injectable({
       providedIn: 'root'  // Provided in the root injector
     })
     export class MyService {
       constructor() { }
     }

3. Constructor Injection:

   • Definition: Dependencies are typically injected into a class via its constructor.
   • Usage: When Angular creates an instance of a class, it looks at the constructor parameters and provides the necessary dependencies.
   • Example: Injecting a service into a component.

     import { Component } from '@angular/core';
     import { MyService } from './my-service.service';
     
     @Component({
       selector: 'app-example',
       templateUrl: './example.component.html'
     })
     export class ExampleComponent {
       constructor(private myService: MyService) {
         // Now you can use myService in this component
       }
     }

Hierarchical Dependency Injection:

1. Hierarchical Injectors:

   • Angular’s DI system is hierarchical, meaning that injectors are organized in a tree structure that parallels the component tree.
   • Each injector can provide dependencies to its child injectors.

2. Injector Hierarchy:

   • Root Injector: Created when the application is bootstrapped. Provides services registered at the application level.
   • Component Injector: Each component has its own injector that can provide dependencies to the component and its children.
   • Module Injector: Specific modules can also have their own injectors.

Example Scenarios:

1. Providing a Service at the Root Level:

   • ProvidedIn: Using providedIn: 'root' ensures the service is a singleton and available application-wide.

     @Injectable({
       providedIn: 'root'
     })
     export class GlobalService {
       constructor() { }
     }

2. Providing a Service in a Module:

   • NgModule Providers: Registering a service in an @NgModule makes it available to all components declared in that module.

     @NgModule({
       providers: [ModuleService]
     })
     export class FeatureModule { }

3. Providing a Service in a Component:

   • Component Providers: Registering a service in a component’s providers array makes it available only to that component and its children.

     @Component({
       selector: 'app-child',
       providers: [LocalService]
     })
     export class ChildComponent {
       constructor(private localService: LocalService) { }
     }

Benefits of Dependency Injection in Angular:

1. Modularity:

   • Promotes loosely coupled and modular code, making it easier to manage and scale.

2. Maintainability:

   • Simplifies the process of updating and refactoring code since dependencies are managed centrally.

3. Testability:

   • Facilitates unit testing by allowing dependencies to be easily mocked or stubbed.

4. Reusability:

   • Encourages the creation of reusable services and components.

In Summary:

Dependency Injection (DI) in Angular is a core design pattern that enhances the modularity, maintainability, and testability of applications. It is implemented through providers, the @Injectable decorator, and constructor injection, with a hierarchical injector system that supports flexible dependency management across different levels of an application. This approach ensures that components and services can efficiently share and manage dependencies in a clean and organized manner.