nestjs-expert

The "Nest can't resolve dependencies" error occurs when providers are not correctly configured or exported across Nest.js modules. You must ensure all required providers are properly registered in the module's provider array and that dependent modules import the originating module to access those exports.

Testing in Nest.js with Jest and Supertest involves creating test modules, mocking dependencies, and configuring test environments. You build a testing module with Nest.createTestingModule, override providers with custom mocks, and use Supertest to run HTTP assertions against your API endpoints.

Nest.js dependency injection handles circular dependencies using forward references. You resolve circular import issues by utilizing the forwardRef() function in your module imports and inject parameters, allowing two services or modules to reference each other without causing instantiation errors.

Integrating TypeORM or Mongoose with a Nest.js backend API requires using the respective Nest.js database packages. You configure the database connection within your root module using forRoot or forRootAsync, define your entities or schemas, and inject the repository or model into your services.

Configuring Passport.js authentication in Nest.js involves implementing custom guards and interceptors to protect routes. You define a Passport strategy, apply the AuthGuard to your controllers, and use interceptors to map or modify the response data before it returns to the client.

Nest.js middleware provider scope conflicts happen when request-scoped providers are incorrectly mixed with singleton-scoped modules. You must align provider scopes across modules and ensure middleware correctly resolves dependencies by checking the injection scope defined in the provider metadata.