Constructor injection in NestJS - Step-by-Step Execution

Concept Flow - Constructor injection

Define Service Class

↓

Add @Injectable Decorator

↓

Define Constructor with Dependency Parameter

↓

NestJS Creates Instance

↓

NestJS Injects Dependency into Constructor

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Service Ready to Use with Dependency

This flow shows how NestJS uses constructor injection to provide dependencies automatically when creating a service instance.

Execution Sample

NestJS

import { Injectable } from '@nestjs/common';

@Injectable()
export class DogsService {}

@Injectable()
export class CatsService {
  constructor(private readonly dogsService: DogsService) {}
}

This code defines a CatsService that receives DogsService via constructor injection.

Execution Table

Step	Action	Evaluation	Result
1	NestJS reads CatsService class	Finds @Injectable decorator	Marks CatsService as injectable
2	NestJS sees constructor parameter dogsService	Type is DogsService	Prepares to inject DogsService instance
3	NestJS creates DogsService instance	DogsService is injectable	DogsService instance ready
4	NestJS calls CatsService constructor	Passes DogsService instance as argument	CatsService instance created with dogsService
5	CatsService ready	dogsService property set	Can use dogsService methods inside CatsService

💡 Constructor injection completes when CatsService instance is created with DogsService dependency injected.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
dogsService	undefined	DogsService instance created	Injected into CatsService constructor	Available as private readonly property

Key Moments - 3 Insights

Why do we add @Injectable() above the service class?

NestJS needs @Injectable() to know it can create and inject this class. Without it, injection won't work (see execution_table step 1).

How does NestJS know what to inject into the constructor?

NestJS uses the constructor parameter type (DogsService) to find or create the right instance to inject (see execution_table step 2 and 3).

What happens if the dependency is missing or not injectable?

NestJS will throw an error during instance creation because it can't resolve the dependency (not shown in table but happens before step 4).

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 3?

A NestJS creates an instance of DogsService

B NestJS calls CatsService constructor

C NestJS marks CatsService as injectable

D NestJS injects DogsService into CatsService

💡 Hint

Check the 'Action' and 'Result' columns at step 3 in the execution_table.

At which step is the DogsService instance passed into CatsService constructor?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look for the step where CatsService constructor is called with DogsService instance.

If we remove @Injectable() from CatsService, what will happen?

A NestJS will still inject DogsService without error

B NestJS will throw an error because CatsService is not injectable

C DogsService will not be created

D CatsService will be created but dogsService will be undefined

💡 Hint

Refer to key_moments about the role of @Injectable() and execution_table step 1.

Concept Snapshot

Constructor Injection in NestJS:
- Use @Injectable() on classes to enable injection.
- Declare dependencies as constructor parameters.
- NestJS creates and injects dependencies automatically.
- This helps keep code clean and testable.
- Missing @Injectable() causes injection errors.

Full Transcript

Constructor injection in NestJS means that when you create a service class, you add the @Injectable decorator so NestJS knows it can create it. Then you add dependencies as parameters in the constructor. When NestJS creates your service, it automatically creates and passes the dependencies into the constructor. This way, your service can use other services easily without manually creating them. If you forget the @Injectable decorator, NestJS will not inject dependencies and will throw an error. This process helps keep your code organized and easy to test.