Overview

Angular services are a fundamental concept in Angular 8, designed to promote reusable code. They are used to organize and share business logic, models, or data and functions across components without duplicating the code. Services are typically singletons within the scope of an Angular application, meaning a single instance of a service is created and shared across components, promoting efficient data management and code reusability.

Key Concepts

1. Singleton Pattern: Services in Angular are created as singletons by default, ensuring that a single instance of a service is used throughout the application.
2. Dependency Injection: Angular leverages dependency injection (DI) to provide required services to components, enhancing modularity and testability.
3. Service Providers: Services need to be registered with an Angular module or component to be accessible. This registration is done in the providers array of the @NgModule() or @Component() decorator.

Common Interview Questions

Basic Level

1. What is an Angular service and why is it used?
2. How do you create and use a service in Angular?

Intermediate Level

3. Explain how dependency injection works in Angular services.

Advanced Level

4. How can you optimize service performance in Angular?

Detailed Answers

1. What is an Angular service and why is it used?

Answer: An Angular service is a class with a specific purpose and provides a method to encapsulate reusable logic or data across various components in an Angular application. It is used to keep the component lean, focusing on presenting data and delegating data access and logic to services. Services help in achieving separation of concerns, enhancing modularity, reusability, and maintainability of the application.

Key Points:
- Promotes code reusability.
- Follows the singleton pattern.
- Enhances component modularity by separating the data layer or business logic from the presentation layer.

Example:

import { Injectable } from '@angular/core';

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

  getGreetings() {
    return 'Hello, Angular 8!';
  }
}

2. How do you create and use a service in Angular?

Answer: To create a service in Angular, you use the @Injectable() decorator to define a class as a service. Then, you can use Angular's dependency injection framework to inject it into components.

Key Points:
- @Injectable() decorator marks a class as available to an injector for instantiation.
- The providedIn property with the value 'root' ensures that the service is available throughout the application.
- Services are injected into components through their constructors.

Example:

import { Injectable } from '@angular/core';

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

  getData() {
    return ['Angular', 'React', 'Vue'];
  }
}

import { Component } from '@angular/core';
import { DataService } from './data.service';

@Component({
  selector: 'app-example',
  template: `<ul><li *ngFor="let item of items">{{item}}</li></ul>`,
})
export class ExampleComponent {
  items: string[];

  constructor(private dataService: DataService) {
    this.items = this.dataService.getData();
  }
}

3. Explain how dependency injection works in Angular services.

Answer: Dependency Injection (DI) in Angular is a design pattern and mechanism to create and deliver parts of an application (dependencies) to different parts of an application that require them. When Angular creates a component, it first asks for the required services and other dependencies. The DI framework then checks if any requested services exist in the injector and, if so, provides them to the component.

Key Points:
- DI decouples the creation of a client's dependencies from the client's behavior, simplifying component testing.
- Angular's injector hierarchy allows for singleton services or multiple instances based on where the service is provided.
- Services can be injected into components, directives, pipes, and other services.

Example:

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class LoggerService {
  log(message: string) {
    console.log(message);
  }
}

import { Component } from '@angular/core';
import { LoggerService } from './logger.service';

@Component({
  selector: 'app-di-example',
  template: `Check the console.`,
})
export class DIExampleComponent {
  constructor(private logger: LoggerService) {
    this.logger.log('Dependency Injection example');
  }
}

4. How can you optimize service performance in Angular?

Answer: Performance optimization for Angular services involves several strategies, such as lazy loading services, using the providedIn property judiciously, leveraging Angular's hierarchical dependency injection system, and minimizing unnecessary external HTTP requests.

Key Points:
- Lazy loading services can significantly reduce the initial load time by splitting services into feature modules loaded only when needed.
- Use the providedIn property to specify where the service should be provided, potentially reducing the size of the root injector and improving application startup time.
- Avoid creating unnecessary instances of services across different modules unless absolutely needed.
- Optimize external data fetch calls by implementing caching mechanisms or using observables to manage and minimize the number of HTTP requests.

Example:

// Example of a lazy-loaded feature module
import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { FeatureComponent } from './feature.component';
import { FeatureService } from './feature.service';

@NgModule({
  declarations: [FeatureComponent],
  imports: [CommonModule],
  providers: [FeatureService] // Service is provided only in the scope of this module
})
export class FeatureModule {}

This example demonstrates providing a service specifically in the scope of a lazy-loaded feature module, optimizing the application's performance by not loading the service until the module is actually needed.