<!--
title: Support flexible logging
domain: app-frameworks
topic: Logging
language: JavaScript
source: fastify/fastify
updated: 
url: https://awesomereviewers.com/reviewers/fastify-support-flexible-logging/
-->

When designing logging interfaces, create flexible APIs that accommodate both standard and custom logging needs. This includes:

1. **Support custom log levels**: Don't assume only standard levels (trace, debug, info, warn, error, fatal) will be used. Always check against `logger.levels` which includes any custom levels.

2. **Allow message format customization**: Provide hooks that let users customize log message content.

3. **Enable level selection based on context**: Let users determine the appropriate log level dynamically based on runtime properties.

Example of a flexible logging interface:

```javascript
// Instead of this limited approach:
childLogger.info({ req: request }, 'incoming request')

// Provide a customization hook that gives control over both level and message:
function createRequestLogMessage(childLogger, req) {
  // User can choose log level based on request properties
  const level = req.headers['x-debug'] ? 'debug' : 'info'
  
  // User can customize the message content
  const message = `Incoming ${req.method} request to ${req.url}`
  
  // User controls both level and formatting
  childLogger[level]({ req: req, custom: true }, message)
}

// Usage
if (createRequestLogMessage) {
  createRequestLogMessage(childLogger, req)
} else {
  childLogger.info({ req: request }, 'incoming request')
}
```

This approach increases flexibility without sacrificing simplicity for common use cases.