# TestingLogger

**Kind:** Class

**Source:** [`packages/testing/services/testing-logger.service.ts`](https://github.com/nestjs/nest/blob/master/packages/testing/services/testing-logger.service.ts#L6)

**Part of:** [Testing](subsystem-packages-testing)

`TestingLogger` is a test-focused logger implementation that provides the standard logging methods used throughout the application: `log`, `warn`, `debug`, `verbose`, and `error`. It allows tests to capture, inspect, or safely handle log output without depending on the production logging infrastructure.

**Extends:** `ConsoleLogger`

## Methods

| Method | Signature | Returns |
|---|---|---|
| `log` | `log(message: string)` | `void` |
| `warn` | `warn(message: string)` | `void` |
| `debug` | `debug(message: string)` | `void` |
| `verbose` | `verbose(message: string)` | `void` |
| `error` | `error(message: string, optionalParams: any[])` | `void` |

## Diagram

```mermaid
graph LR
  Test[Test Suite] --> Logger[TestingLogger]
  Logger --> Log[log()]
  Logger --> Warn[warn()]
  Logger --> Debug[debug()]
  Logger --> Verbose[verbose()]
  Logger --> Error[error()]
  Log --> Assertions[Test Assertions]
  Warn --> Assertions
  Debug --> Assertions
  Verbose --> Assertions
  Error --> Assertions
```

## Usage

```ts
import { TestingLogger } from '@your-package/testing';

describe('ExampleService', () => {
  it('logs an expected message', () => {
    const logger = new TestingLogger();

    logger.log('Processing started');
    logger.warn('Optional configuration is missing');
    logger.debug('Request payload received');
    logger.verbose('Additional diagnostic details');
    logger.error('Processing failed');

    // Use the testing logger with the service under test.
    // const service = new ExampleService(logger);
  });
});
```

## AI Coding Instructions

- Use `TestingLogger` when unit or integration tests need a logger dependency instead of the production logger.
- Call the method that matches the intended log level: `log` for normal messages, `warn` for recoverable issues, and `error` for failures.
- Keep test logging deterministic; avoid relying on timestamps, environment-specific values, or external output destinations.
- When adding logger-compatible services, preserve support for all standard methods exposed by `TestingLogger`.
- Prefer assertions against expected logging behavior when logs represent important operational or error-handling outcomes.

## How it works

## `TestingLogger`

`TestingLogger` is a public class that extends Nest’s `ConsoleLogger`. Its constructor initializes the inherited logger with the context string `"Testing"`. [packages/testing/services/testing-logger.service.ts:3-9]

## Relationships

- IMPORTS → `ConsoleLogger`