Why services are needed in Angular - Performance Evidence

Performance: Why services are needed

MEDIUM IMPACT

This concept affects how efficiently data and logic are shared across components, impacting rendering speed and user interaction responsiveness.

Sharing data and logic between multiple components

Angular

Create a singleton Angular service that fetches data once and shares it via Observables to ComponentA and ComponentB.

Data is fetched once, cached, and shared, reducing network load and avoiding repeated rendering.

📈 Performance GainSingle network request and fewer change detection runs, improving INP.

Sharing data and logic between multiple components

Angular

ComponentA and ComponentB each fetch the same data independently inside ngOnInit() without a shared service.

This causes duplicate HTTP requests and redundant processing, increasing load time and CPU usage.

📉 Performance CostTriggers multiple network requests and duplicate change detection cycles.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Duplicate data fetching in components	Multiple redundant DOM updates	Multiple reflows due to repeated change detection	Higher paint cost from frequent updates	[X] Bad
Shared data via singleton service	Minimal DOM updates triggered by shared Observables	Single reflow per data change	Lower paint cost due to fewer updates	[OK] Good

Rendering Pipeline

Services centralize data and logic, reducing redundant DOM updates and network calls. Components subscribe to service data streams, triggering fewer change detection cycles.

→Network

→JavaScript Execution

→Change Detection

→Rendering

⚠️ BottleneckExcessive network requests and repeated change detection caused by duplicated logic in components.

Core Web Vital Affected

INP

This concept affects how efficiently data and logic are shared across components, impacting rendering speed and user interaction responsiveness.

Optimization Tips

1Use singleton services to share data and logic across components.

2Avoid fetching the same data multiple times in different components.

3Centralize state management in services to reduce redundant rendering.

Performance Quiz - 3 Questions

What is a main performance benefit of using Angular services to share data?

A Increases bundle size significantly

B Reduces duplicate network requests and redundant rendering

C Triggers more change detection cycles

D Blocks rendering until all components load

DevTools: Performance

How to check: Record a session while interacting with components that fetch data. Look for multiple network requests and repeated scripting or rendering tasks.

What to look for: Multiple identical network requests and frequent scripting tasks indicate redundant data fetching; fewer requests and smoother scripting indicate good service use.

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