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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:

// 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.