@jalik/logger

Easy and customizable logging for your apps.

Features

• Enabling/disabling logging anytime
• Naming of the logger
• Filtering of log events using a custom function
• Support multiple outputs (works sequentially)
• Customizing format of log events
• Ability to pass a context for each log event
• Ability to set a default context for log events
• Use of standards logging levels (debug, info, warning, error, fatal)
• TypeScript declarations ♥

Sandbox

Play with the lib here: https://codesandbox.io/s/jalik-logger-default-example-75o5hx

Installing

npm i -P @jalik/logger

yarn add @jalik/logger

Creating a logger

import { Logger } from '@jalik/logger';

const logger = new Logger();

Which is equivalent to a production default setup:

import {
  Logger,
  INFO
} from '@jalik/logger';

const logger = new Logger({
  // Enable the logger
  active: true,
  // Only log events with an INFO level or more
  level: INFO,
  // Set the name of this logger (auto-generated if not set)
  name: 'logger_123456789',
  // Set logging outputs
  outputs: [
    // Output logs to the console
    consoleOutput()
  ]
});

Logging levels

The following levels are available (ordered from the less important to the most important).

• debug: used for debugging messages
• info: used for informational messages
• warn: used for warning messages
• error: used for error messages
• fatal: used for fatal error messages

They can be imported individually.

import {
  DEBUG,
  INFO,
  WARN,
  ERROR,
  FATAL
} from '@jalik/logger';

Or they can be imported as an array.

import { levels } from '@jalik/logger';

Logging messages

debug(message: string, context?: any)

import { Logger } from '@jalik/logger';

const logger = new Logger({ name: 'main' });
const a = 2;
const b = 4;
const result = a + b;

// Logs a message with a context
logger.debug(`result = ${result}`, { a, b });
// or without context
logger.debug(`result = ${result}`);

info(message: string, context?: any)

import { Logger } from '@jalik/logger';

const logger = new Logger({ name: 'main' });
const bootTime = 1337;

// Log the message with a context
logger.info(`Application started in ${bootTime} ms`, { bootTime, tags: ['boot'] });
// or without context
logger.info(`Application started in ${bootTime} ms`);

warn(message: string, context?: any)

import { Logger } from '@jalik/logger';

const logger = new Logger({ name: 'main' });
const diskUsage = 93.6;

// Log the message with a context
logger.warn('Disk usage is above 90%', { diskUsage });
// or without context
logger.warn('Disk usage is above 90%');

error(message: string, context?: any)

import { Logger } from '@jalik/logger';

const logger = new Logger({ name: 'main' });
const error = new Error('Forbidden');

// Log the message with a context
logger.error('Forbidden', { error });
// or without context
logger.error('Forbidden');
// or simply (it will use error.message)
logger.error(error);

fatal(message: string, context?: any)

import { Logger } from '@jalik/logger';

const logger = new Logger({ name: 'main' });
const error = new Error('app crashed');

// Log the message with a context
logger.fatal('app crashed', { error });
// or without context
logger.fatal('app crashed');
// or simply (it will use error.message)
logger.fatal(error);

log(level: string, message: string, context?: any)

This is the "low level" function called by other logging functions.

import {
  Logger,
  INFO
} from '@jalik/logger';

const logger = new Logger({ name: 'main' });
const ipAddress = '6.6.6.6';

// Logs an informational message with a context.
logger.log(INFO, `The IP address ${ipAddress} has failed to login 3 times`, { ipAddress });

Enabling or disabling logging

Logging is enabled by default if you don't set active: false in Logger options. However, you can change logging status at anytime with the setActive(boolean) method.

setActive(boolean)

import { Logger } from '@jalik/logger';

const logger = new Logger({
  // Enable logger on production environment only.
  active: process.env.NODE_ENV === 'PRODUCTION'
});

// To check if the logger is active.
logger.isActive();

// Disable logger after 30 seconds.
setTimeout(() => {
  // Anything that is logged after this line will be ignored.
  logger.setActive(false);
  logger.info('Sky is blue');
}, 30000)

isActive(): boolean

This method tells you if the logger is enabled.

Setting a default context

It is possible to set defaultContext when creating the logger.
This context will be passed to all log events and may be overridden for each log.

import { Logger } from '@jalik/logger';

const logger = new Logger({
  defaultContext: {
    host: process.env.HOST
  }
});

// then logging a message will automatically use the default context.
logger.info('Application started.');

// you can even add a context over a default context (attributes will be merged and/or replaced).
logger.info('Something happened', { tag: 'something-event' });

Filtering log events

You can filter the logs that are processed by using the filter option when creating a logger.

import {
  DEBUG,
  Logger
} from '@jalik/logger';

const cronFilter = (event) => {
  return (event.context && event.context.tag === 'cron') || /cron/g.test(event.message)
}

const logger = new Logger({
  level: DEBUG,
  filter: cronFilter
});

// this will be logged.
logger.info('Cron jobs executed.', { tag: 'cron' });

// this will not be logged.
logger.info('Application started.');

Logging outputs

A logger can be configured with several outputs, all of them are executed sequentially. By default, a logger is configured to output messages to the console with consoleOutput().

consoleOutput(options)

The console output displays logs in the console (browser and nodejs).

import {
  Logger,
  consoleOutput
} from '@jalik/logger';

const logger = new Logger({
  name: 'main',
  outputs: [
    consoleOutput(),
  ],
});

logger.info('Hello World', { number: 42 });
// will log:
// 2021-05-27T02:40:06.957Z INFO [main] : Hello World ; {"number":42}

By default, consoleOutput() uses the defaultFormatter() function to format log events, but you can provide your own formatter.

import {
  Logger,
  consoleOutput,
} from '@jalik/logger';

function customFormatter(event) {
  // format: "LEVEL [LOGGER] : MESSAGE"
  const { level, logger, message } = event;
  return [level.toUpperCase(), `[${logger}]`, ':', message].join(' ');
}

const logger = new Logger({
  name: 'main',
  outputs: [
    consoleOutput({ formatter: customFormatter }),
  ],
});

logger.info('Hello World', { number: 42 });
// will log:
// INFO [main] : Hello World

fileOutput(options)

The file output writes log events to a file, so it can only be used on NodeJS.

import {  Logger,} from '@jalik/logger';
import fileOutput from '@jalik/logger/dist/outputs/fileOutput.js'

const logger = new Logger({
  name: 'main',
  outputs: [
    fileOutput({
      // the logs destination file
      path: 'logs.txt',
      // the formatter to use
      formatter: JSON.stringify,
      // improve performances by flushing (writing) logs at interval
      // instead of writing logs every time
      flushInterval: 1000
    }),
  ],
});

logger.info('Hello World', { number: 42 });
// will log:
// {"timestamp":1682982410055,"level":"INFO","logger":"main","message":"Hello World","context":{"number":42}}

Changelog

History of releases is in the changelog on github.

License

The code is released under the MIT License.