slf

Simple Logging Facade for Node.js applications.

Install

npm install slf

Quick start

import { ConsoleLogger, LoggerFactory } from 'slf';

LoggerFactory.setFactory(ConsoleLogger);

const log = LoggerFactory.getLogger('app:startup');

log.info('Boot complete');
log.warn('Configuration is missing optional key: %s', 'featureX');

Log levels

Available levels:

• debug
• info
• warn
• error
• critical

Set a minimum level when configuring the factory:

import { ConsoleLogger, Level, LoggerFactory } from 'slf';

LoggerFactory.setFactory(ConsoleLogger, Level.Info);

Or set the level via environment variable:

SLF_LOG_LEVEL=debug

Supported values should be lower case: debug, info, warn, error, critical.

Logger API

Create a logger:

import { LoggerFactory } from 'slf';

const log = LoggerFactory.getLogger('service:payments');

Write logs:

log.debug('Debug message');
log.info('Info message');
log.warn('Warn message');
log.error('Error message');

Generic method:

log.log('warn', 'Low disk space: %d%%', 9);
log.log('This defaults to info');

Middleware

You can intercept and modify log events:

import { LoggerFactory } from 'slf';

const prefix = (event, next) => {
  event.params = ['[my-service]', ...event.params];
  next(null, event);
};

LoggerFactory.use(prefix);
LoggerFactory.use(prefix); // The same function is registered only once.

LoggerFactory.remove(prefix); // Returns true when removed, false if not registered.

Writing a custom driver

A factory receives one or more events:

import { Event, LoggerFactory } from 'slf';

const customFactory = (...events: Event[]) => {
  for (const event of events) {
    // send event to your logging backend
  }
};

LoggerFactory.setFactory(customFactory);

Each event has:

• name: logger name
• level: lowercase log level string
• params: original log arguments
• timeStamp: epoch milliseconds