# ADR-026: Standardized Client-Side Structured Logging

**Date**: 2025-12-14

**Status**: Proposed

## Context

Following the standardization of backend logging in `ADR-004`, it is clear that our frontend components also require a consistent logging strategy. Currently, components either use `console.log` directly or a simple wrapper, but without a formal standard, this can lead to inconsistent log formats and difficulty in debugging user-facing issues.

While the frontend does not have the concept of a "request-scoped" logger, the principles of structured, context-rich logging are equally important for:

1. **Effective Debugging**: Understanding the state of a component or the sequence of user interactions that led to an error.
2. **Integration with Monitoring Tools**: Sending structured logs to services like Datadog, Sentry, or LogRocket allows for powerful analysis and error tracking in production.
3. **Clean Test Outputs**: Uncontrolled logging can pollute test runner output, making it difficult to spot actual test failures.

An existing client-side logger at `src/services/logger.client.ts` already provides a simple, structured logging interface. This ADR formalizes its use as the application standard.

## Decision

We will adopt a standardized, application-wide structured logging policy for all client-side (React) code.

**1. Mandatory Use of the Global Client Logger**: All frontend components, hooks, and services **MUST** use the global logger singleton exported from `src/services/logger.client.ts`. Direct use of `console.log`, `console.error`, etc., is discouraged.

**2. Pino-like API for Structured Logging**: The client logger mimics the `pino` API, which is the standard on the backend. It supports two primary call signatures:

- `logger.info('A simple message');`
- `logger.info({ key: 'value' }, 'A message with a structured data payload');`

The second signature, which includes a data object as the first argument, is **strongly preferred**, especially for logging errors or complex state.

**3. Mocking in Tests**: All Jest/Vitest tests for components or hooks that use the logger **MUST** mock the `src/services/logger.client.ts` module. This prevents logs from appearing in test output and allows for assertions that the logger was called correctly.

### Example Usage

**Logging an Error in a Component:**

```typescriptreact
// In a React component or hook
import { logger } from '../services/logger.client';
import { notifyError } from '../services/notificationService';

const fetchData = async () => {
  try {
    const data = await apiClient.getData();
    return data;
  } catch (err) {
    // Log the full error object for context, along with a descriptive message.
    logger.error({ err }, 'Failed to fetch component data');
    notifyError('Something went wrong. Please try again.');
  }
};
```

**Mocking the Logger in a Test File:**

```typescript
// In a *.test.tsx file
import { vi } from 'vitest';

// Mock the logger at the top of the test file
vi.mock('../services/logger.client', () => ({
  logger: {
    info: vi.fn(),
    warn: vi.fn(),
    error: vi.fn(),
    debug: vi.fn(),
  },
}));

describe('MyComponent', () => {
  beforeEach(() => {
    vi.clearAllMocks(); // Clear mocks between tests
  });

  it('should log an error when fetching fails', async () => {
    // ... test setup to make fetch fail ...

    // Assert that the logger was called with the expected structure
    expect(logger.error).toHaveBeenCalledWith(
      expect.objectContaining({ err: expect.any(Error) }), // Check for the error object
      'Failed to fetch component data', // Check for the message
    );
  });
});
```

## Consequences

### Positive

**Consistency**: All client-side logs will have a predictable structure, making them easier to read and parse.
**Debuggability**: Errors logged with a full object (`{ err }`) capture the stack trace and other properties, which is invaluable for debugging.
**Testability**: Components that log are easier to test without polluting CI/CD output. We can also assert that logging occurs when expected.
**Future-Proof**: If we later decide to send client-side logs to a remote service, we only need to modify the central `logger.client.ts` file instead of every component.

### Negative

**Minor Boilerplate**: Requires importing the logger in every file that needs it and mocking it in every corresponding test file. However, this is a small and consistent effort.