A Node.js CommonJS/ES6 module with a generic console logger class

This project provides a TypeScript Node.js CommonJS/ES6 module with a re-entrant console logger suitable for REPL/server/multi-instance use cases.

Note: Compatibility with legacy CommonJS is required until VS Code extensions will be updated to import ES6 modules.

The open source project is hosted on GitHub as xpack/logger-ts.

Maintainer & developer info

This page documents how to use this module in an user application. For maintainer information, see the separate README-MAINTAINER page.

Prerequisites

A recent Node.js (>=16.0.0), since the TypeScript code is compiled into ECMAScript 2020 code, and the tests use ES6 modules.

Install

The module is available as @xpack/logger from the public npmjs repository; it can be added as a dependency to any TypeScript or JavaScript project with npm install:

npm install --save @xpack/logger@latest

The module does not provide any executables, and generally there are no reasons to install it globally.

User info

This section is intended for those who plan to use this module in their own projects.

The @xpack/logger module can be imported into both TypeScript and JavaScript Node.js code

In TypeScript and ECMAScript modules, use import:

import { Logger } from '@xpack/logger'

In JavaScript with CommonJS, use require():

const { Logger } = request('@xpack/logger')

The module can be included in any application and the class can be used directly or a custom class can be derived from it for a custom behaviour.

The typical use case is to create an instance of the Logger object, then log messages at different levels:

const log = new Logger({
  level: 'info'
})

log.info('hello') // Displayed on stdout.
log.debug('world') // Ignored.

In more complex use cases, the log level can be tested and the (possibly) long operations be performed only if necessary.

Log levels

The following strings are recognised as valid level names:

export type LogLevel =
  'silent' | 'error' | 'warn' | 'info' | 'verbose' | 'debug' | 'trace' | 'all'

Internally the string levels are converted to integer values, and these integers are used in comparisons. Higher values mean more verbosity.

Delaying setting the log level

There are cases when the logger must be created very early in the life cycle of an application, even before it is practically possible to determine the log level.

For these cases, if the logger is created without a log level, it is set to a preliminary state, and all log lines are stored in an internal buffer**, until the log level is set, when the buffer is walked and the lines are processed.

const log = new Logger()
log.trace('...') // Not shown immediately
log.level = 'trace' // Set level and show the buffered messages.

log.info('hello') // Displayed on stdout.
log.debug('world') // Ignored.

Output methods

The following methods are available to log messages:

• always (message: any = '', ...args: any[]): void
• error (message: any = '', ...args: any[]): void
• error (message: Error): void
• output (message: any = '', ...args: any[]): void
• warn (message: any = '', ...args: any[]): void
• info (message: any = '', ...args: any[]): void
• verbose (message: any = '', ...args: any[]): void
• debug (message: any = '', ...args: any[]): void
• trace (message: any = '', ...args: any[]): void

Reference

For more details on the available class definitions, including all methods, accessors, properties, etc, please see the TypeDoc reference pages.

Known problems

• none

Status

The @xpack/logger module is fully functional and stable.

The main client for this module is the xpm CLI application.

Tests

The module is tested with 100% coverage and CI tested on every push via GitHub Actions.

Compatibility notices

According to semver rules:

Major version X (X.y.z | X > 0) MUST be incremented if any backwards incompatible changes are introduced to the public API.

v6.0.0

The project was migrated to TypeScript and the code is compiled into CommonJS modules, with an ES6 wrapper, and can be consumed by both TypeScript and JavaScript packages.

There were also some minor internal renames, but this should not be a problem.

The function isLevel(level) was added.

v5.0.0

For consistency reasons, hasLevel was changed from a method to an accessor.

Internally, the log level starts as undefined instead of the string 'undefined', as in previous versions.

This should not be a problem, given that the method to check if the level was set is via hasLevel().

v4.0.0

The code was migrated to TypeScript.

The migration itself should not introduce any incompatibilities, actually it should be fairly compatible with the latest v3.x, but, for just in case, the safer path was to consider it a major release.

v3.0.0

All isXyx functions (returning a boolean related to the log level) were changed to accessors.

v2.0.0

The logger constructor was changed to use a generic parameters object.

If upgrading from previous versions, change the syntax from:

const log = new Logger(console, 'info')

to:

const log = new Logger({
  console,
  level: 'info'
})

License

The original content is released under the MIT License, with all rights reserved to Liviu Ionescu.