Modules

A module is a class with the @Module() decorator that groups the code of one part of the application: controllers, services and settings. The blog will have modules for posts, comments and users, and the root AppModule brings them together.

The posts module

Create a module with the CLI:

nest g module posts

The file src/posts/posts.module.ts appears:

// src/posts/posts.module.ts
import { Module } from '@nestjs/common';
 
@Module({})
export class PostsModule {}

The module is empty for now. The CLI has already connected it to the root module — src/app.module.ts now has imports: [PostsModule].

The four module properties

Property	What goes in it
`controllers`	Controllers that handle this module's requests
`providers`	Services and other classes that NestJS creates itself
`imports`	Other modules whose services this one uses
`exports`	Its own services that other modules may use

Once the module has a controller and a service, it looks like this:

// src/posts/posts.module.ts
import { Module } from '@nestjs/common';
import { PostsController } from './posts.controller';
import { PostsService } from './posts.service';
 
@Module({
  controllers: [PostsController],
  providers: [PostsService],
})
export class PostsModule {}

A service for another module

By default a service is visible only inside its own module. Comments need to check that a post exists, so the posts module exports its service:

// src/posts/posts.module.ts
@Module({
  controllers: [PostsController],
  providers: [PostsService],
  exports: [PostsService],
})
export class PostsModule {}

And the comments module imports the posts module:

// src/comments/comments.module.ts
import { Module } from '@nestjs/common';
import { PostsModule } from '../posts/posts.module';
import { CommentsService } from './comments.service';
 
@Module({
  imports: [PostsModule],
  providers: [CommentsService],
})
export class CommentsModule {}

Now CommentsService can receive PostsService — the lesson about services shows how.

The root module

// src/app.module.ts
import { Module } from '@nestjs/common';
import { PostsModule } from './posts/posts.module';
import { CommentsModule } from './comments/comments.module';
 
@Module({
  imports: [PostsModule, CommentsModule],
})
export class AppModule {}

At startup NestJS begins with AppModule and walks through all imports. The terminal shows lines like PostsModule dependencies initialized — the module was found and built.

Common mistake

A module exports a service, but the other module forgets to import it. At startup NestJS prints Nest can't resolve dependencies of the CommentsService (?) and names the module it searched. Both steps are needed: exports in the owning module and imports in the module that uses the service. Do not add someone else's service to your own providers — that creates a second, independent copy.

Thanks for reading!