How to avoid circular dependencies in NestJS

NestJS is a framework that excels in separating application concerns through its architecture, which allows the creation of services as dependencies for controllers. However, one of the challenges developers face is dealing with circular dependencies, which occur when two or more modules depend on each other either directly or indirectly. This can prevent code from compiling as the NestJS dependency injection system relies on metadata that cannot resolve circular references. The text explains how to avoid tight coupling, which violates SOLID principles, by refactoring code to eliminate circular dependencies or using forward references as a workaround. An illustrative example is provided where circular dependencies between UserService and FileService are resolved by introducing a new ProfilePictureService. Additionally, it highlights the use of the forwardRef() utility function to manage unavoidable circular dependencies, ensuring code compilation without errors. The discussion emphasizes the importance of understanding dependency injection in NestJS to effectively manage and mitigate issues related to circular dependencies.