Why services are needed in Angular - See It in Action

Why services are needed

📖 Scenario: You are building a simple Angular app that shows user information on multiple components. You want to share the same user data between these components without repeating code or creating confusion.

🎯 Goal: Build an Angular service to hold user data and use it in two components to display the same information.

📋 What You'll Learn

Create a service to store user data

Create two components that use the service to get user data

Show the user name in both components

Use Angular dependency injection to provide the service

💡 Why This Matters

🌍 Real World

Sharing user data or settings across multiple parts of an app without duplicating code.

💼 Career

Understanding services and dependency injection is essential for building scalable Angular applications.

1

Create the user data service

Create an Angular service called UserService with a public property userName set to the string 'Alice'.

Angular

// Create UserService with userName property
// Your code here

Hint

Use @Injectable decorator with providedIn: 'root' to make the service available app-wide.

2

Create the first component using the service

Create an Angular component called FirstComponent that injects UserService in its constructor and has a public property userName set to the service's userName. Display {{ userName }} in the component template.

Angular

import { Component } from '@angular/core';
import { UserService } from './user.service';

@Component({
  selector: 'app-first',
  template: `User: {{ userName }}`
})
export class FirstComponent {
  // Your code here
}

Hint

Inject the service in the constructor and assign userName from the service.

3

Create the second component using the service

Create an Angular component called SecondComponent that injects UserService in its constructor and has a public property userName set to the service's userName. Display {{ userName }} in the component template.

Angular

import { Component } from '@angular/core';
import { UserService } from './user.service';

@Component({
  selector: 'app-second',
  template: `User: {{ userName }}`
})
export class SecondComponent {
  // Your code here
}

Hint

Repeat the same pattern as FirstComponent to use the service in SecondComponent.

4

Use both components in the app template

Add the selectors <app-first></app-first> and <app-second></app-second> inside the main app component template to show both components on the page.

Angular

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

@Component({
  selector: 'app-root',
  template: `
    
  `
})
export class AppComponent {}

Hint

Use the component selectors as HTML tags inside the app component template.

Practice

1. Why do Angular apps use services instead of putting all code inside components?

easy

A. To make components load slower

B. To share data and logic across components easily

C. To avoid using TypeScript

D. To write more HTML code

Solution

1. Step 1: Understand the role of services

   Services hold shared data and logic that multiple components can use.

2. Step 2: Compare with components

   Components focus on showing the UI, not on sharing data or logic.

3. Final Answer:

   To share data and logic across components easily -> Option B

4. Quick Check:

   Services share data and logic = A [OK]

Hint: Services share logic, components show UI [OK]

Common Mistakes:

• Thinking services slow down the app
• Believing services replace HTML
• Confusing services with components

2. Which is the correct way to inject a service into an Angular component constructor?

easy

A. constructor() { this.myService = MyService; }

B. constructor(myService = new MyService()) {}

C. constructor(public myService) {}

D. constructor(private myService: MyService) {}

Solution

1. Step 1: Recall Angular dependency injection syntax

   Angular injects services by declaring them as private/public in the constructor parameters.

2. Step 2: Check each option

   constructor(private myService: MyService) {} uses correct syntax with private and type annotation. Others create instances manually or miss type.

3. Final Answer:

   constructor(private myService: MyService) {} -> Option D

4. Quick Check:

   Use private with type in constructor = D [OK]

Hint: Use private and type in constructor for services [OK]

Common Mistakes:

• Manually creating service instances
• Missing type annotation
• Assigning service without new keyword

3. Given this service and component code, what will be logged?

export class CounterService {
  count = 0;
  increment() { this.count++; }
}

@Component({ selector: 'app-one', template: '' })
export class OneComponent {
  constructor(private counter: CounterService) {}
  ngOnInit() { this.counter.increment(); console.log(this.counter.count); }
}

@Component({ selector: 'app-two', template: '' })
export class TwoComponent {
  constructor(private counter: CounterService) {}
  ngOnInit() { this.counter.increment(); console.log(this.counter.count); }
}

medium

A. 1 and 2

B. 1 and 1

C. 0 and 0

D. 2 and 2

Solution

1. Step 1: Understand service instance sharing

   Angular provides the same service instance to both components if the service is singleton.

2. Step 2: Trace increments and logs

   OneComponent increments count to 1 and logs 1. Then TwoComponent increments to 2 and logs 2.

3. Final Answer:

   1 and 2 -> Option A

4. Quick Check:

   Shared service count increments = C [OK]

Hint: Singleton service shares state across components [OK]

Common Mistakes:

• Assuming separate service instances per component
• Ignoring increment effect order
• Thinking count resets each time

4. What is wrong with this service usage in a component?

@Injectable({ providedIn: 'root' })
export class DataService {
  data = [];
}

@Component({ selector: 'app-test', template: '' })
export class TestComponent {
  dataService: DataService;
  constructor(private dataService: DataService) {
  }
}

medium

A. Service should be injected, not created with new

B. DataService must not have data property

C. Component must not have constructor

D. Service must be provided in component, not root

Solution

1. Step 1: Identify service instantiation method

   The component creates a new service instance manually using new, bypassing Angular's injector.

2. Step 2: Understand Angular DI pattern

   Services should be injected via constructor parameters to get the singleton instance managed by Angular.

3. Final Answer:

   Service should be injected, not created with new -> Option A

4. Quick Check:

   Inject services via constructor, not new [OK]

Hint: Never use new for services, always inject [OK]

Common Mistakes:

• Creating service instances manually
• Ignoring Angular dependency injection
• Misunderstanding providedIn usage

5. You want to share user login status across multiple components and keep UI code simple. Which approach best uses Angular services?

hard

A. Use local variables in components without services

B. Store login status in each component separately

C. Create a UserService with login status and methods, inject it in components

D. Write login logic inside each component's template

Solution

1. Step 1: Identify need for shared state

   Login status must be shared and consistent across components.

2. Step 2: Choose best practice

   Using a UserService to hold login state and methods keeps components simple and shares data.

3. Final Answer:

   Create a UserService with login status and methods, inject it in components -> Option C

4. Quick Check:

   Use service to share state and logic [OK]

Hint: Use services to share app state and logic [OK]

Common Mistakes:

• Duplicating state in components
• Putting logic in templates
• Avoiding services for shared data