Custom providers in NestJS - Mini Project: Build & Apply

Creating and Using Custom Providers in NestJS

📖 Scenario: You are building a simple NestJS application that needs to provide a greeting message. Instead of hardcoding the greeting, you want to use a custom provider to supply the greeting text. This approach helps keep your code flexible and easy to change later.

🎯 Goal: Build a NestJS module that uses a custom provider named GREETING to supply a greeting string. Then inject this provider into a service to return the greeting message.

📋 What You'll Learn

Create a custom provider named GREETING with the value 'Hello, NestJS!'

Create a service called GreetingService that injects the GREETING provider

Add a method getGreeting() in GreetingService that returns the injected greeting string

Register the custom provider and the service in a module called AppModule

💡 Why This Matters

🌍 Real World

Custom providers in NestJS allow you to inject values, classes, or factories that are not standard services. This is useful for configuration values, constants, or third-party libraries.

💼 Career

Understanding custom providers is essential for building scalable and maintainable NestJS applications, a skill often required for backend developer roles using Node.js and NestJS.

1

Create the custom provider with the greeting string

Create a constant called GREETING and set it to the string 'Hello, NestJS!'. Then create a provider object called greetingProvider with the key provide set to GREETING and useValue set to 'Hello, NestJS!'.

NestJS

# Create the GREETING constant and greetingProvider object
# Your code here

Need a hint?

Remember, a custom provider in NestJS is an object with provide and useValue keys.

2

Create the GreetingService that injects the custom provider

Create a class called GreetingService. Use the @Injectable() decorator. Add a constructor that injects the GREETING provider using @Inject(GREETING) and stores it in a private readonly variable called greeting.

NestJS

export const GREETING = 'GREETING';

export const greetingProvider = {
  provide: GREETING,
  useValue: 'Hello, NestJS!'
};

// Create GreetingService with @Injectable and inject GREETING
# Your code here

Need a hint?

Use @Inject(GREETING) in the constructor parameter to inject the custom provider.

3

Add a method to return the greeting string

Inside the GreetingService class, add a method called getGreeting() that returns the private greeting string.

NestJS

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

export const GREETING = 'GREETING';

export const greetingProvider = {
  provide: GREETING,
  useValue: 'Hello, NestJS!'
};

@Injectable()
export class GreetingService {
  constructor(@Inject(GREETING) private readonly greeting: string) {}

  // Add getGreeting() method here
  # Your code here
}

Need a hint?

The method should simply return the injected greeting string.

4

Register the provider and service in AppModule

Create a class called AppModule with the @Module() decorator. Inside the decorator, add providers array containing greetingProvider and GreetingService. Export the AppModule class.

NestJS

import { Injectable, Inject, Module } from '@nestjs/common';

export const GREETING = 'GREETING';

export const greetingProvider = {
  provide: GREETING,
  useValue: 'Hello, NestJS!'
};

@Injectable()
export class GreetingService {
  constructor(@Inject(GREETING) private readonly greeting: string) {}

  getGreeting(): string {
    return this.greeting;
  }
}

// Create AppModule and register greetingProvider and GreetingService
# Your code here

Need a hint?

Use the @Module() decorator to register providers and services.