Angular · Theory

Dependency Injection

Theory

14/43

Dependency Injection

DI means Angular creates the object and passes it in the constructor. constructor(private marks: MarksService). You don’t call new. Providers tell Angular how to make it.

providedIn: 'root' → one singleton. providedIn a component → new instance for that component tree. Use root for MarksService. Use component provider only when you want isolation.

Trap: circular inject (A needs B needs A). Split a third helper. Another trap: forgetting to provide a non-root service and getting NullInjectorError.

Diagram

  Angular DI
     │ constructor(private marks)
     ▼
  MarksService (root)

Exam tip

constructor inject + providedIn. NullInjectorError meaning.

Example

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

@Injectable({ providedIn: "root" })
export class UserService {
  getName() {
    return "Asha";
  }
}

Dependency Injection — services hold shared logic; Angular DI injects them into components.

Short notes

DefAngular constructs and injects. No new.
Ruleroot singleton vs component-scoped.
TrapNullInjectorError / circular DI.

Angular · Theory

Dependency Injection

← All stacks

Theory

14/43

Dependency Injection

DI means Angular creates the object and passes it in the constructor. constructor(private marks: MarksService). You don’t call new. Providers tell Angular how to make it.

providedIn: 'root' → one singleton. providedIn a component → new instance for that component tree. Use root for MarksService. Use component provider only when you want isolation.

Trap: circular inject (A needs B needs A). Split a third helper. Another trap: forgetting to provide a non-root service and getting NullInjectorError.

Diagram

  Angular DI
     │ constructor(private marks)
     ▼
  MarksService (root)

Exam tip

constructor inject + providedIn. NullInjectorError meaning.

Example

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

@Injectable({ providedIn: "root" })
export class UserService {
  getName() {
    return "Asha";
  }
}

Dependency Injection — services hold shared logic; Angular DI injects them into components.

Short notes

DefAngular constructs and injects. No new.
Ruleroot singleton vs component-scoped.
TrapNullInjectorError / circular DI.

Dependency Injection

Example

Short notes

Questions

Dependency Injection

Example

Short notes

Questions