loglevel-plugin-prefix

What is loglevel-plugin-prefix?

The loglevel-plugin-prefix npm package is a plugin for the loglevel logging library that allows you to add customizable prefixes to your log messages. This can be useful for adding context to your logs, such as timestamps, log levels, or custom tags.

What are loglevel-plugin-prefix's main functionalities?

Basic Prefixing

This feature allows you to add a basic prefix to your log messages. By default, it adds the log level as a prefix.

const log = require('loglevel');
const prefix = require('loglevel-plugin-prefix');
prefix.apply(log);
log.info('This is an info message');

Custom Prefix Format

This feature allows you to customize the format of the prefix. You can include a timestamp, log level, and logger name in the prefix.

const log = require('loglevel');
const prefix = require('loglevel-plugin-prefix');
prefix.apply(log, {
  template: '[%t] %l (%n):',
  timestampFormatter: date => date.toISOString(),
  levelFormatter: level => level.toUpperCase(),
  nameFormatter: name => name || 'global'
});
log.info('This is an info message');

Custom Prefix Function

This feature allows you to define a custom function to format the prefix. This gives you complete control over how the prefix is generated.

const log = require('loglevel');
const prefix = require('loglevel-plugin-prefix');
prefix.apply(log, {
  format(level, name, timestamp) {
    return `${timestamp} [${level}] ${name}:`;
  }
});
log.info('This is an info message');

Other packages similar to loglevel-plugin-prefix

winston

Winston is a versatile logging library for Node.js that supports multiple transports, custom formats, and log levels. It offers more advanced features compared to loglevel-plugin-prefix, such as log rotation and integration with various logging services.

bunyan

Bunyan is another logging library for Node.js that provides a JSON-based logging format. It includes features like log levels, serializers, and streams. Bunyan is more focused on structured logging compared to the prefixing capabilities of loglevel-plugin-prefix.

pino

Pino is a fast and low-overhead logging library for Node.js. It supports JSON logging, log levels, and custom serializers. Pino is designed for performance and is more lightweight compared to loglevel-plugin-prefix.

README

loglevel-plugin-prefix

Plugin for loglevel message prefixing.

Installation

npm install loglevel-plugin-prefix

API

This plugin is under active development and should be considered as an unstable. No guarantees regarding API stability are made. Backward compatibility is guaranteed only by path releases.

reg(loglevel)

This method registers plugin for loglevel. This method must be called at least once before any call to the apply method. Repeated calls to this method are ignored.

Parameters

loglevel - the root logger, imported from loglevel module

apply(logger, options)

This method applies the plugin to the logger. Before using this method, the reg method must be called, otherwise a warning will be logged. From the next release, the call apply before reg will throw an error.

Parameters

logger - any logger of loglevel

options - an optional configuration object

var defaults = {
  template: '[%t] %l:',
  levelFormatter: function (level) {
    return level.toUpperCase();
  },
  nameFormatter: function (name) {
    return name || 'root';
  },
  timestampFormatter: function (date) {
    return date.toTimeString().replace(/.*(\d{2}:\d{2}:\d{2}).*/, '$1');
  },
  format: undefined
};

Plugin formats the prefix using template option as a printf-like format. The template is a string containing zero or more placeholder tokens. Each placeholder token is replaced with the value from loglevel messages parameters. Supported placeholders are:

• %l - level of message
• %n - name of logger
• %t - timestamp of message

The levelFormatter, nameFormatter and timestampFormatter is a functions for formatting corresponding values.

Alternatively, you can use format option. This is a function that receives formatted values (level, name, timestamp) and should returns a prefix string.

If both format and template are present in the configuration, the template parameter is ignored. When both these parameters are missing in the configuration, the inherited behavior is used.

Usage

Browser directly

<script src="https://unpkg.com/loglevel/dist/loglevel.min.js"></script>
<script src="https://unpkg.com/loglevel-plugin-prefix@^0.8/dist/loglevel-plugin-prefix.min.js"></script>

<script>
  var logger = log.noConflict();
  var prefixer = prefix.noConflict();
  prefixer.reg(logger);
  prefixer.apply(logger);
  logger.warn('prefixed message');
</script>

Output

[16:53:46] WARN: prefixed message

Node

const chalk = require('chalk');
const log = require('loglevel');
const prefix = require('loglevel-plugin-prefix');

const colors = {
  TRACE: chalk.magenta,
  DEBUG: chalk.cyan,
  INFO: chalk.blue,
  WARN: chalk.yellow,
  ERROR: chalk.red,
};

prefix.reg(log);
log.enableAll();

prefix.apply(log, {
  format(level, name, timestamp) {
    return `${chalk.gray(`[${timestamp}]`)} ${colors[level.toUpperCase()](level)} ${chalk.green(`${name}:`)}`;
  },
});

prefix.apply(log.getLogger('critical'), {
  format(level, name, timestamp) {
    return chalk.red.bold(`[${timestamp}] ${level} ${name}:`);
  },
});

log.trace('trace');
log.debug('debug');
log.getLogger('critical').info('Something significant happened');
log.log('log');
log.info('info');
log.warn('warn');
log.error('error');

Output

Custom options

const log = require('loglevel');
const prefix = require('loglevel-plugin-prefix');

prefix.reg(log);
log.enableAll();

prefix.apply(log, {
  template: '[%t] %l (%n) static text:',
  levelFormatter(level) {
    return level.toUpperCase();
  },
  nameFormatter(name) {
    return name || 'global';
  },
  timestampFormatter(date) {
    return date.toISOString();
  },
});

log.info('%s prefix', 'template');

const fn = (level, name, timestamp) => `[${timestamp}] ${level} (${name}) static text:`;

prefix.apply(log, { format: fn });

log.info('%s prefix', 'functional');

prefix.apply(log, { template: '[%t] %l (%n) static text:' });

log.info('again %s prefix', 'template');

Output

[2017-05-29T12:53:46.000Z] INFO (global) static text: template prefix
[2017-05-29T12:53:46.000Z] INFO (global) static text: functional prefix
[2017-05-29T12:53:46.000Z] INFO (global) static text: again template prefix

Option inheritance

const log = require('loglevel');
const prefix = require('loglevel-plugin-prefix');

prefix.reg(log);
log.enableAll();

log.info('root');

const chicken = log.getLogger('chicken');
chicken.info('chicken');

prefix.apply(chicken, { template: '%l (%n):' });
chicken.info('chicken');

prefix.apply(log);
log.info('root');

const egg = log.getLogger('egg');
egg.info('egg');

const fn = (level, name) => `${level} (${name}):`;

prefix.apply(egg, { format: fn });
egg.info('egg');

prefix.apply(egg, {
  levelFormatter(level) {
    return level.toLowerCase();
  },
});
egg.info('egg');

chicken.info('chicken');
log.info('root');

Output

root
chicken
INFO (chicken): chicken
[16:53:46] INFO: root
[16:53:46] INFO: egg
INFO (egg): egg
info (egg): egg
INFO (chicken): chicken
[16:53:46] INFO: root