Working with NestJS, a progressive Node.js framework for building efficient and scalable server-side applications, developers leverage the powerful Dependency Injection (DI) system that the framework provides. However, encountering an error where an injected service turns out to be undefined can halt the development, and it’s crucial to understand and solve this issue in an effective manner.
Understanding the Error
In NestJS, when a service, or any provider for that matter, is injected into a constructor of a component such as a controller or another service, the framework takes responsibility for creating an instance of the injected class and passing it as a parameter into the constructor. However, things may go awry due to incorrect module configuration, incorrect scope of the service, or perhaps improperly specified custom provider tokens. When such an issue arises, it’s pivotal to carefully inspect parts of the code where the service is being defined (module files) and where it’s being injected (consumer files).
Module Configuration and Providers Array
Begin by examining the module file where the service is declared. Every service in NestJS must be a part of a module’s providers array. Without this inclusion, the service won’t be a part of the module’s context, hence the DI system won’t be able to find and inject it accordingly. Furthermore, if the service is to be shared across modules, it must be included in the exports array of the module file. Fixing the issue here could involve simply adding the missing service to the appropriate arrays.
Service Decorator and Scopes
Another potential source of the issue lies within the service class itself. The class must be decorated with @Injectable() which marks it as a provider. The scope of the service could also introduce complications; a singleton scope (the default) vs a transient or request scope can lead to different instances being injected. Make sure the scope is correctly configured and is consistent where the service is injected.
Custom Provider Tokens
In cases where custom providers are defined, it’s vital to ascertain that tokens used in the useClass, useValue, or useFactory properties within the providers’ configuration match those used for injection. A mismatch between declaration and injection tokens will result in an undefined injected service.
Complete Code Example
The code example below illustrates a properly configured service and its injection into a consumer component. Assumptions include usage of the latest syntax of NestJS and TypeScript compatibility.
import { Module, Injectable } from '@nestjs/common';
@Injectable()
class MyService {
getHello(): string {
return 'Hello World!';
}
}
@Module({
providers: [MyService],
exports: [MyService]
})
class MyModule {}
@Injectable()
class MyConsumer {
constructor(private myService: MyService) {}
greet(): string {
return this.myService.getHello();
}
}
@Module({
imports: [MyModule],
controllers: [],
providers: [MyConsumer],
})
class MyConsumerModule {}
// Start the application
async function bootstrap() {
const app = await NestFactory.create(MyConsumerModule);
await app.listen(3000);
}
bootstrap();
Following the example above, if the MyService is undefined within MyConsumer, the error could be resolved by ensuring that MyService is in the providers array of MyModule, and that MyModule is imported in MyConsumerModule. This example offers a solid template that exemplifies proper usage and configuration of services in NestJS. By applying the same principles, developers can solve many cases where services are injected as undefined, ensuring smooth and seamless development workflows.
