Injectable decorator in NestJS - Step-by-Step Execution

Concept Flow - Injectable decorator

Define class

↓

Add @Injectable decorator

↓

NestJS registers class as provider

↓

Class can be injected into constructors

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NestJS creates instance when needed

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Use instance in dependent classes

Shows how adding @Injectable marks a class for NestJS to manage and inject where needed.

Execution Sample

NestJS

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

@Injectable()
export class CatsService {
  getCats() { return ['Tom', 'Jerry']; }
}

Defines a service class marked with @Injectable so NestJS can inject it.

Execution Table

Step	Action	Evaluation	Result
1	Read class CatsService	Class defined	Class CatsService created
2	Apply @Injectable decorator	Decorator marks class	Class registered as provider
3	NestJS scans providers	Finds CatsService	CatsService added to DI container
4	Another class requests CatsService	Constructor injection	NestJS creates CatsService instance
5	Call getCats()	Method runs	Returns ['Tom', 'Jerry']
6	End of flow	No more actions	Execution stops

💡 All steps complete; class is injectable and usable via NestJS DI

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
CatsService	undefined	Registered as provider	Instance created	Instance used

Key Moments - 2 Insights

Why do we need the @Injectable decorator on a class?

Without @Injectable, NestJS does not register the class as a provider, so it cannot inject it. See execution_table step 2 where the decorator marks the class.

What happens when another class requests CatsService in its constructor?

NestJS looks up CatsService in its container and creates an instance to inject. See execution_table step 4 for this injection process.

Visual Quiz - 3 Questions

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

A Another class requests CatsService

B CatsService method getCats() is called

C NestJS registers CatsService as a provider

D The class CatsService is defined

💡 Hint

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

At which step does NestJS create an instance of CatsService?

A Step 4

B Step 5

C Step 2

D Step 1

💡 Hint

Look for 'Instance created' in the 'Result' column in execution_table

If we remove @Injectable decorator, which step would fail?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Without @Injectable, NestJS cannot register the class as provider (step 3)

Concept Snapshot

@Injectable decorator marks a class as a provider
NestJS registers it in the dependency injection container
Allows class instances to be injected into constructors
Enables reuse and easy testing
Without it, injection fails
Use on services, repositories, and providers

Full Transcript

The @Injectable decorator in NestJS marks a class so the framework knows to manage it as a provider. When NestJS starts, it scans for classes with @Injectable and registers them in its dependency injection container. Later, when another class needs this provider, NestJS creates an instance and injects it automatically. This process allows easy sharing and reuse of services. Without @Injectable, NestJS will not recognize the class for injection, causing errors. The execution flow starts with defining the class, applying the decorator, registering the provider, injecting the instance, and finally using the class methods.