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Caterpillar is the ultimate logging system for Node.js, based on transform streams you can log to it and pipe the output off to different locations, including some pre-made ones. Caterpillar also supports log levels according to the RFC standard, as well as line, method, and file fetching for messages. You can even use it in web browsers with the Browser Transform.



  • Use: require('caterpillar')
  • Install: npm install --save caterpillar


  • Use: require('caterpillar')
  • Install: npm install --save caterpillar
  • CDN URL: //


  • Use: require('caterpillar')
  • Install: ender add caterpillar


Example with the Filter and Human transforms

// Import
var level   = process.argv.indexOf('-d') === -1 ? 6 : 7;
var logger  = require('caterpillar').createLogger({level:level});
var filter  = require('caterpillar-filter').createFilter();
var human   = require('caterpillar-human').createHuman();

// Where to output?
if ( process.title === 'browser' ) {
    // Include the browser compatibility layer
    var browser = require('caterpillar-browser').createBrowser();

    // Pipe to filter to human to browser
else {
    // Pipe to filter to human to stdout

    // If we are debugging, then write the original logger data to debug.log
    if ( level === 7 ) {

// Log messages
logger.log('emergency', 'this is level 0');
logger.log('emerg', 'this is level 0');
logger.log('alert', 'this is level 1');
logger.log('critical', 'this is level 2');
logger.log('crit', 'this is level 2');
logger.log('error', 'this is level 3');
logger.log('err', 'this is level 3');
logger.log('warning', 'this is level 4');
logger.log('warn', 'this is level 4');
logger.log('notice', 'this is level 5');
logger.log('note', 'this is level 5');
logger.log('info', 'this is level 6');
logger.log('default', 'this is level 6');
logger.log('debug', 'this is level 7');
logger.log('this is level 6, the default level');

Result with log level 6 (info):

Result with log level 7 (debug):

Result with log level 7 (debug) in the browser with the Browser Transform

Transform API, extends stream.Transform

new (require('caterpillar').Transform)(config)
  • Methods
    • constructor(config?) create our new instance with the config, config is optional
    • pipe(child) pipe our stream to the child, also sync our config to it
    • setConfig(config) set the configuration and emit the config event
    • getConfig() get the configuration
    • format(entry) format the caterpillar logger entry
  • Configuration
    • none by default
  • Events
    • config(config) emitted once our configuration has updated

Logger API, extends Transform API

new (require('caterpillar').Logger)(config)
  • Methods

    • constructor(config?) create our new instance with the config, config is optional
    • log(args...) write a log message, the first argument is suppose to be the level (will use the default level if it isn't)
    • format(level, args...) create a log entry ready for writing to the logger stream, output is as follows:

          "args": ["this is emergency and is level 0"],
          "date": "2013-04-25T10:18:25.722Z",
          "levelNumber": 0,
          "levelName": "emergency",
          "line": "59",
          "method": "Task.fn",
          "file": "/Users/balupton/Projects/caterpillar/out/test/caterpillar-test.js"
    • getLevelNumber(name) get the level number for the level name

    • getLevelName(number) get the level name for the level number
    • getLevelInfo(nameOrNumber) get the level name and number for either a level name or number
    • getLineInfo() get the file, method, and line that the log method was called on
  • Configuration

    • lineOffset the amount of lines to offset when doing our line detection, useful for wrappers, defaults to 0
    • levels the level names and their associated number, also includes default for when no level was specified, defaults to:

          emergency: 0,
          alert: 1,
          critical: 2,
          error: 3,
          warning: 4,
          notice: 5,
          info: 6,
          debug: 7,
          emerg: 0,
          crit: 2,
          err: 3,
          warn: 4,
          note: 5,
          default: 6
  • Events

    • only those inherited


Discover the change history by heading on over to the file.



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Licensed under the incredibly permissive MIT license

Copyright © 2012+ Bevry Pty Ltd (
Copyright © 2011 Benjamin Lupton (

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