How dependency injection works in Angular - Visual Walkthrough

Concept Flow - How dependency injection works in Angular

Component or Service needs a dependency

↓

Angular checks the injector tree

↓

Find provider for the dependency

↓

Create instance if not created

↓

Inject instance into the requester

↓

Component/Service uses the dependency

Angular looks for the requested dependency in its injector tree, creates it if needed, and provides it to the component or service that asked for it.

Execution Sample

Angular

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

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

export class AppComponent {
  private logger = inject(LoggerService);
  constructor() { this.logger.log('App started'); }
}

This code shows Angular injecting LoggerService into AppComponent using the inject() function.

Execution Table

Step	Action	Injector State	Dependency Found	Instance Created	Injected Into
1	AppComponent requests LoggerService	Empty	No	No	No
2	Angular checks root injector	Empty	Yes	No	No
3	LoggerService instance created	LoggerService instance created	Yes	Yes	No
4	LoggerService instance injected into AppComponent	LoggerService instance created	Yes	Yes	AppComponent
5	AppComponent calls logger.log()	LoggerService instance created	Yes	Yes	AppComponent

💡 Dependency injected and used; execution stops after AppComponent construction.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
LoggerService instance	undefined	created	created	created
AppComponent.logger	undefined	undefined	LoggerService instance	LoggerService instance

Key Moments - 3 Insights

Why does Angular create only one instance of LoggerService?

Because LoggerService is provided in 'root', Angular creates a singleton instance in the root injector as shown in execution_table step 3.

What happens if the dependency is not found in the injector?

Angular throws an error because it cannot provide the requested dependency, but in this example, the dependency is found at step 2.

How does Angular know where to inject the dependency?

Angular uses the inject() function or constructor parameters to know where to inject, demonstrated in step 4 where the instance is injected into AppComponent.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the LoggerService instance created?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Check the 'Instance Created' column in the execution_table.

According to the variable tracker, when does AppComponent.logger get assigned the LoggerService instance?

A After Step 4

B After Step 3

C After Step 2

D At Start

💡 Hint

Look at the 'AppComponent.logger' row in variable_tracker after Step 4.

If LoggerService was not provided in 'root', what would happen during injection?

A Angular would create a new instance anyway

B Angular would inject null

C Angular would throw an error at step 2

D Angular would skip injection

💡 Hint

Refer to key_moments about what happens if dependency is not found.

Concept Snapshot

Angular Dependency Injection (DI):
- Components or services request dependencies.
- Angular checks injector tree for providers.
- Creates singleton instances if needed.
- Injects instances where requested.
- Use @Injectable and inject() for DI.
- Root injector provides app-wide singletons.

Full Transcript

Dependency injection in Angular works by components or services asking for dependencies. Angular looks into its injector tree to find a provider for the requested dependency. If the provider exists and no instance is created yet, Angular creates one. Then Angular injects this instance into the requester. For example, a LoggerService marked with @Injectable({providedIn: 'root'}) is created once in the root injector. When AppComponent uses inject(LoggerService), Angular provides the same LoggerService instance. This process ensures efficient reuse and easy testing. If Angular cannot find a provider, it throws an error. This flow is shown step-by-step in the execution table and variable tracker.