@filipgorny/logger

Lightweight, colorful logging package with strategy pattern for flexible log output.

Features

• Strategy pattern - Easily switch between logging strategies
• Type-safe - Full TypeScript support
• Multiple log levels - DEBUG, INFO, WARN, ERROR
• Contextual logging - Add metadata to logs
• Child loggers - Create loggers with inherited context
• Colorized console output - Beautiful, readable logs with custom colors
• Test-friendly - Automatically suppresses error logs in test environments
• Clean format - Simple [HH:MM:SS] message format
• Future-ready - Prepared for Datadog integration

Installation

npm install @filipgorny/logger
# or
pnpm add @filipgorny/logger

Usage

Basic Usage

import { createLogger, LogLevel } from "@filipgorny/logger";

const logger = createLogger("my-service", LogLevel.INFO);

logger.debug("Debugging info"); // Cyan
logger.info("Server started"); // Yellow
logger.warn("High memory usage"); // Magenta/Pink
logger.error("Connection failed"); // Red

Output Format

Logs are displayed in a clean, colorized format:

[08:46:40] Server started         # Yellow text, white timestamp
[08:46:45] High memory usage      # Pink text, white timestamp
[08:46:50] Connection failed      # Red text, white timestamp

Color Scheme

• Timestamp [HH:MM:SS] - White
• DEBUG - Cyan
• INFO - Yellow
• WARN - Magenta (Pink)
• ERROR - Red

Custom Strategy

import { Logger, LogLevel, LogStrategy } from "@filipgorny/logger";

class CustomStrategy implements LogStrategy {
  log(entry: LogEntry): void {
    // Your custom logging logic
  }
}

const logger = new Logger(new CustomStrategy(), {
  service: "my-service",
  level: LogLevel.INFO,
});

Child Loggers

const logger = createLogger("api-service");
const requestLogger = logger.child({ requestId: "123", userId: "abc" });

requestLogger.info("Processing request"); // Includes requestId and userId

Switching Strategies at Runtime

import { ConsoleLogStrategy } from "@filipgorny/logger";

// Start with console
logger.setStrategy(new ConsoleLogStrategy());

// Switch to custom strategy
logger.setStrategy(new CustomStrategy());

Test Environment

The logger automatically detects test environments (NODE_ENV=test or Jest) and suppresses ERROR level console output to keep test output clean.

API

createLogger(serviceName, level?)

Create a new logger instance.

• serviceName: Name of the service (e.g., 'llm-service')
• level: Minimum log level (default: LogLevel.INFO)

Logger Methods

• debug(message, context?) - Debug level logging
• info(message, context?) - Info level logging
• warn(message, context?) - Warning level logging
• error(message, error?, context?) - Error level logging
• child(context) - Create a child logger with inherited context
• setStrategy(strategy) - Change the logging strategy
• setLevel(level) - Change the minimum log level

Log Levels

• DEBUG - Detailed debugging information
• INFO - General informational messages (default)
• WARN - Warning messages
• ERROR - Error messages

Strategies

ConsoleLogStrategy (Default)

Logs to console with colorized output and clean timestamp format.

Custom Strategy

Implement the LogStrategy interface:

import { LogStrategy, LogEntry } from "@filipgorny/logger";

class MyCustomStrategy implements LogStrategy {
  log(entry: LogEntry): void {
    // Your custom logging logic
    console.log(`[${entry.service}] ${entry.message}`);
  }
}

Advanced Features

Table Logging

Display data in a beautiful table format:

logger.table(
  [
    { name: "John", age: 30, city: "NYC" },
    { name: "Jane", age: 25, city: "LA" },
  ],
  [
    { key: "name", header: "Name" },
    { key: "age", header: "Age" },
    { key: "city", header: "City" },
  ],
);

License

MIT