standard-log

standard-log is a powerful and extensible logging library.

Key features

• Support micro-app with isolated log system.
• Create multiple loggers for different part of your application.
• Provide multiple log levels out of the box.
• Support custom log levels.
• Three levels of log level controls: log system, logger, and log method.
• Send logs to multiple reporters, e.g. console, file, memory, or remote service.
• Custom formatting and filtering for each reporter.
• Security focus. After the system is created, it cannot be changed.

Usage

There are two ways to use standard-log: global or standalone.

Standalone is the preferred way. You call createStandardLog() to get an isolated log system.

import { createStandardLog } from 'standard-log'

const standardLog = createStandarLog(/* options */)

const log = standardLog.getLogger('my-app')

log.error('error message')
log.warn('warn message')
log.info('info message')
// by default debug() will not be printed
// because production mode defaults log level to `info`
log.debug('debug message')

// If the message is consume resource to generage,
// you can use `log.on()` so that it will be called only
// if the level is met.
log.on(logLevels.trace, () => prettify(someValue))

(above is logged with standard-log-color)

You can configure a logger by doing getLogger(id, options):

options.level: number:

Log level of this logger.

options.writeTo: string | RegExp | ((reporterId: string) => boolean):

Only log to specific reporter(s).

Log Level

standard-log log level defaults to logLevels.info. It comes with many log levels out of the box:

log.emergency('msg')
log.alert('msg')
log.critical('msg')
log.error('msg')
log.warn('msg')
log.notice('msg')
log.info('msg')
log.debug('msg')
log.trace('msg')
log.planck('msg')

When sending logs to console, they are mapped to info, warn, error, and debug based on severity.

You can also add your own custom levels:

import { createStandardLog } from 'standard-log'

const standardLog = createStandardLog({
  customLevels: {
    'important': logLevel.warn + 1,
    'silly': logLevel.debug + 1
  }
})

const log = standardLog.getLogger('custom')
log.important('this is an important message')
log.silly('oh silly')

Reporters

Besides printing the logs to console, you can use different reporters to save the logs in memory, file, service, or others.

import { createStandardLog, createConsoleLogReporter, createMemoryLogReporter } from 'standard-log'

createStandardLog({
  reporters: [createConsoleLogReporter(), createMemoryLogReporter()]
})

Some reporters allow you to format the logs and/or filter them. Using the console log reporter as an example:

import { createConsoleLogReporter } from 'standard-log'

createConsoleLogReporter({
  formatter: (entry) => [...],
  filter: (entry) => entry.args.every(arg => arg !== 'secret')
})

Here are some additional reporters:

• standard-log-color
• standard-log-file (TODO)
• standard-log-syslog

Suppress log

You can temporarily suppress logs by suppressLogs(). This allows you to disable downstream logs to reduce log noises.

import { getLogger, suppressLogs } from 'standard-log'

const log = getLogger('some logger')

suppressLogs(() => log.info('not logged'), log)

Testing

During test, you should use createStandardLogForTest which includes a MemoryReporter to capture the logs.

import { createStandardLogForTest } from 'standard-log/testing'

test('your test', () => {
  const standardLog = createStandardLogForTest()
  yourApp.standardLog = standardLog

// do your thing...

  const messages = standardLog.reporter.getLogMessage() // or getLogMessageWithLevel()
  // validate the message if you want to
})

Global usage

You can also call getLogger() to get a logger and log away:

import { getLogger } from 'standard-log'

const log = getLogger('my-lib')
log.info('log away')

It internally creates a global instance of standard-log. To configure this global instance, use configGlobal():

import { configGlobal } from 'standard-log'

configGlobal({ logLevel: logLevels.info, reporters: [ ... ] })

This global instance does not support custom levels. And it will emit a warning when configGlobal() is called more than once. Typically, configGlobal() should be called by the application, and it happens only once. But in micro-app situation, the library can be shared, and each application can call configGlobal()

In general, using this global instance should be avoided. It is the main driving force for 9.0.