Common errors

During your development with NestJS, you may encounter various errors as you learn the framework.

"Cannot resolve dependency" error

Probably the most common error message is about Nest not being able to resolve dependencies of a provider. The error message usually looks something like this:

Nest can't resolve dependencies of the <provider> (?). Please make sure that the argument <unknown_token> at index [<index>] is available in the <module> context.

Potential solutions:
- If <unknown_token> is a provider, is it part of the current <module>?
- If <unknown_token> is exported from a separate @Module, is that module imported within <module>?
  @Module({
    imports: [ /* the Module containing <unknown_token> */ ]
  })

The most common culprit of the error, is not having the provider in the module's providers array. Please make sure that the provider is indeed in the providers array and following standard NestJS provider practices.

There are a few gotchas, that are common. One is putting a provider in an imports array. If this is the case, the error will have the provider's name where <module> should be.

If you run across this error while developing, take a look at the module mentioned in the error message and look at its providers. For each provider in the providers array, make sure the module has access to all of the dependencies. Often times, providers are duplicated in a "Feature Module" and a "Root Module" which means Nest will try to instantiate the provider twice. More than likely, the module containing the provider being duplicated should be added in the "Root Module"'s imports array instead.

"Circular dependency" error

Occasionally you'll find it difficult to avoid circular dependencies in your application. You'll need to take some steps to help Nest resolve these. Errors that arise from circular dependencies look like this:

Nest cannot create the <module> instance.
The module at index [<index>] of the <module> "imports" array is undefined.

Potential causes:
- A circular dependency between modules. Use forwardRef() to avoid it. Read more: https://docs.nestjs.com/fundamentals/circular-dependency
- The module at index [<index>] is of type "undefined". Check your import statements and the type of the module.

Scope [<module_import_chain>]
# example chain AppModule -> FooModule

Circular dependencies can arise from both providers depending on each other, or typescript files depending on each other for constants, such as exporting constants from a module file and importing them in a service file. In the latter case, it is advised to create a separate file for your constants. In the former case, please follow the guide on circular dependencies and make sure that both the modules and the providers are marked with forwardRef.