A module is a class annotated with a
@Module() decorator. The
@Module() decorator provides metadata that Nest makes use of to organize the application structure.
Each application has at least one module, a root module. The root module is the starting point Nest uses to build the application graph - the internal data structure Nest uses to resolve module and provider relationships and dependencies. While very small applications may theoretically have just the root module, this is not the typical case. We want to emphasize that modules are strongly recommended as an effective way to organize your components. Thus, for most applications, the resulting architecture will employ multiple modules, each encapsulating a closely related set of capabilities.
@Module() decorator takes a single object whose properties describe the module:
|the providers that will be instantiated by the Nest injector and that may be shared at least across this module|
|the set of controllers defined in this module which have to be instantiated|
|the list of imported modules that export the providers which are required in this module|
|the subset of |
The module encapsulates providers by default. This means that it's impossible to inject providers that are neither directly part of the current module nor exported from the imported modules. Thus, you may consider the exported providers from a module as the module's public interface, or API.
CatsService belong to the same application domain. As they are closely related, it makes sense to move them into a feature module. A feature module simply organizes code relevant for a specific feature, keeping code organized and establishing clear boundaries. This helps us manage complexity and develop with SOLID principles, especially as the size of the application and/or team grow.
To demonstrate this, we'll create the
info Hint To create a module using the CLI, simply execute the
$ nest g module catscommand.
Above, we defined the
CatsModule in the
cats.module.ts file, and moved everything related to this module into the
cats directory. The last thing we need to do is import this module into the root module (the
AppModule, defined in the
Here is how our directory structure looks now:
In Nest, modules are singletons by default, and thus you can share the same instance of any provider between multiple modules effortlessly.
Every module is automatically a shared module. Once created it can be reused by any module. Let's imagine that we want to share an instance of the
CatsService between several other modules. In order to do that, we first need to export the
CatsService provider by adding it to the module's
exports array, as shown below:
Now any module that imports the
CatsModule has access to the
CatsService and will share the same instance with all other modules that import it as well.
As seen above, Modules can export their internal providers. In addition, they can re-export modules that they import. In the example below, the
CommonModule is both imported into and exported from the
CoreModule, making it available for other modules which import this one.
A module class can inject providers as well (e.g., for configuration purposes):
However, module classes themselves cannot be injected as providers due to circular dependency .
If you have to import the same set of modules everywhere, it can get tedious. Unlike in Nest, Angular
providers are registered in the global scope. Once defined, they're available everywhere. Nest, however, encapsulates providers inside the module scope. You aren't able to use a module's providers elsewhere without first importing the encapsulating module.
When you want to provide a set of providers which should be available everywhere out-of-the-box (e.g., helpers, database connections, etc.), make the module global with the
@Global() decorator makes the module global-scoped. Global modules should be registered only once, generally by the root or core module. In the above example, the
CatsService provider will be ubiquitous, and modules that wish to inject the service will not need to import the
CatsModule in their imports array.
info Hint Making everything global is not a good design decision. Global modules are available to reduce the amount of necessary boilerplate. The
importsarray is generally the preferred way to make the module's API available to consumers.
The Nest module system includes a powerful feature called dynamic modules. This feature enables you to easily create customizable modules that can register and configure providers dynamically. Dynamic modules are covered extensively here. In this chapter, we'll give a brief overview to complete the introduction to modules.
Following is an example of a dynamic module definition for a
info Hint The
forRoot()method may return a dynamic module either synchronously or asynchronously (i.e., via a
This module defines the
Connection provider by default (in the
@Module() decorator metadata), but additionally - depending on the
options objects passed into the
forRoot() method - exposes a collection of providers, for example, repositories. Note that the properties returned by the dynamic module extend (rather than override) the base module metadata defined in the
@Module() decorator. That's how both the statically declared
Connection provider and the dynamically generated repository providers are exported from the module.
If you want to register a dynamic module in the global scope, set the
global property to
warning Warning As mentioned above, making everything global is not a good design decision.
DatabaseModule can be imported and configured in the following manner:
If you want to in turn re-export a dynamic module, you can omit the
forRoot() method call in the exports array: