NestJS "Cannot read properties of undefined" from a DI provider in CI

A method call failed because an injected dependency was undefined at the moment it was used. In Nest this usually means a circular dependency, a missing forwardRef, or accessing a dependency before construction finished.

What this error means

A request or test throws "TypeError: Cannot read properties of undefined (reading 'find')" from a service method, where the undefined value is an injected provider.

NestJS

TypeError: Cannot read properties of undefined (reading 'findOne')
    at CatsService.get (/app/dist/cats/cats.service.js:14:34)

Common causes

A circular dependency left the provider undefined

When two providers depend on each other without forwardRef, one is undefined when the other is constructed.

A dependency used before initialization

Accessing an injected provider in a field initializer or in the constructor body before Nest finishes wiring leaves it undefined.

How to fix it

Resolve the circular dependency

Check whether the undefined provider is part of a cycle.
Wrap both sides with forwardRef(() => Other).
Or extract the shared logic to break the cycle.

cats.service.ts

constructor(
  @Inject(forwardRef(() => UsersService))
  private readonly users: UsersService,
) {}

Use dependencies after construction

Access injected providers inside methods or lifecycle hooks, not in field initializers that run before injection completes.

How to prevent it

Avoid provider cycles; use forwardRef only when unavoidable.
Do not use injected providers in field initializers.
Unit test services with a compiled TestingModule to catch undefined deps.