Can you explain what cyclic dependencies are in NestJS and how they can be avoided?

Cyclic dependencies in NestJS occur when two or more modules or services depend on each other directly or through a chain of dependencies, creating a closed loop. This leads to errors during dependency injection, as NestJS cannot resolve such a cycle correctly.

For example, if ModuleA imports ModuleB, and ModuleB in turn imports ModuleA, a cyclic dependency arises.

To avoid cyclic dependencies, you can:

• Use forwardRef() — a special NestJS function that allows deferring the resolution of a dependency and breaking the cycle.
• Review the architecture and extract common dependencies into a separate module that will be imported into both modules.
• Use interfaces and abstractions to reduce direct dependencies.

Example with forwardRef():

@Module({
  imports: [forwardRef(() => ModuleB)],
  providers: [ServiceA],
  exports: [ServiceA],
})
export class ModuleA {}

@Module({
  imports: [forwardRef(() => ModuleA)],
  providers: [ServiceB],
  exports: [ServiceB],
})
export class ModuleB {}

This way, NestJS can resolve dependencies correctly despite the cyclic reference.