Next-auth 5.0.0-beta.26 introduced a change that is causing a type error in AuthError:

[everspader]

This commit introduced a change that makes the type property of AuthError an internal property, causing a type error when trying to access the type property in a situation like this:

...
} catch (error) {
    if (error instanceof AuthError) {
      switch (error.type) {
        case "CredentialsSignin":
          return { error: "Invalid credentials" };
        default:
          return { error: "Something went wrong" };
      }
    }
  }

This change is not mentionded in the changelog here so I am wondering what is the purpose of such change?

[Kitwaii]

I do not know the reason either, but it seems that AuthError has just been redesigned to get a clearer access to the error type, message and stack trace.

With that, that is easier to change the sign-in title and message thrown by Auth.js on the sign-in action

// Custom error class you can throw in your `auth.ts` file
export class UserSignInError extends CredentialsSignin {
        // With that, we can also add more precised information to handle error cases
	public readonly httpErrorCode: number

	public constructor(httpErrorCode: number, message: string) {
		super(message)
		this.httpErrorCode = httpErrorCode
		// This will overrides the error title instead of `CredentialsSignin` - and can be dynamic as a parameter too -
		this.name = 'SignInError'
	}
}

On usage, type has just been replaced by name

} catch(error) {
    if (error instanceof AuthError) {
        switch (error.name) {
            case 'CredentialsSignin':
                return { error: 'Invalid credentials' }
            case 'SignInError':
                if (error instance of UserSignInError) {
                    // Here, we can now send clearer error code if we want 
                    return { error: `User has failed to login, as the request returned HTTP ${error.httpErrorCode}` }
                }
                break
            default:
                return { error: 'Something went wrong' }
      }
    }
}