2.0.0-beta.19 • Public • Published

ulog v2.0.0-beta.19

The Universal Logger

npm license travis mind BLOWN



The logger for javascript applications

ulog is the logger for Javascript applications. It's universal, meaning it runs everywhere. You can use ulog in your Express server application running on Node JS just as well as in your React single page application running in the browser. It just works.



Ulog marries the feature sets from debug and loglevel and adds some of it's own!

Feature   debug   loglevel   ulog  
Footprint 3.2 kB 1.4 kB 2.7 kB
Debug mode ✓ (1)
Configurable ✓ (2)
Dynamic config
Custom outputs
Preserves callstack
Configurable format
Custom formats
Add-ons / Mods
Lazy loading
Anylogger support ✓ (3) ✓ (3)

(1) emulated with levels (2) in browser only (3) via an adapter

Try it

Have a look at the interactive tutorial. It's the fastest way to get a quick taste of ulog.

Compare it

Want to check how ulog measures up to it's competitors? Check out these loggers side-by-side:


npm i -S anylogger ulog

Add to entry point

In the entry point of your application import ulog:


import "ulog"


In your code, import anylogger and use it to create loggers and do logging:

import anylogger from 'anylogger'
const log = anylogger('my-app')
log('Logging is easy!')

This way, your code is decoupled from ulog and if you ever want to switch to another logging library, you will be able to do so without having to change any of that code.

Anylogger support

anylogger is a logging facade that allows code to use logging without getting coupled to a specific logging system. You can use that code with any logging system out there. ulog has anylogger support built in. For other loggers, adapters are available.

The logger for libraries

When we write a library, we install ulog as a development dependency so the library remains decoupled from ulog.

Install ulog as a dev dependency

Install anylogger as a regular dependency and ulog as a dev dependency:

npm install --save anylogger && npm install --save-dev ulog

In our tests:


import `ulog`

In our library code:


import anylogger
const log = anylogger('my-lib')
log('Logging is easy')

Script tag

If you want, you can import ulog with a script tag:

<script src="https://unpkg.com/ulog@2.0.0-beta.19/ulog.min.js"></script>
<!-- publishes to `self.anylogger` and `self.ulog`. -->
<!-- lazy loads ulog.lazy.min.js on demand. -->
<script src="myscript.js"></script>


var log = anylogger('my-module')
log('Logging is easy!')


If you want the file for the browser to include in your project yourself, you can download it from here.

ulog.min.js lazy loads ulog.lazy.min.js on demand, so make sure to include both files in your download

Full bundle, no lazy loading

I recommend to use a bundler instead. Loading lots of script tags is inefficient and hard to manage. Also see the section on lazy loading with webpack

Why ulog

The two most popular logging libraries on NPM at the moment are debug and loglevel. They are both great loggers, but neither of them completely satisfied my requirements for a logging library.

debug allows for namespaced debug logging, where each logger has a name. Whether these loggers output debug logging is configurable, though not dynamic, requiring a restart before changes take effect. It's simplicity makes debug an excellent choice for debug logging (as it's name implies), but it lacks support for log levels, so if you want to log error messages for example, you end up needing another library for that. It offers nicely formatted (and even colored!) log messages, but because of that mangles the call stack, which is a huge downside in the browser imho. It's not very extensible, basically being a monolith.

loglevel also supports namespaced logging and it does offer log levels. It's configurable via localStorage but not via environment variables and just like debug requires a restart before configuration changes take effect. By default, it leaves your call stack alone, making the filename/line number entries in the browser console that much more useful. It does not offer alternative log destinations or formatters out of the box. It can be extended via plugins and there are some good plugins out there, but it's base feature set is coded as a monolith, so you cannot easily remove features. You probably won't have to though as it weighs only 1.4kB.

Both these loggers lack the ability to configure the logger from the querystring, which I found to be a very desirable feature for web development as it allows you to create a URL that has the log config embedded, which you can then send to other developers or users etc. E.g: https://example.com/page?log=debug.

What I want is a logging library that combines the best aspects of both these loggers and adds the features that I miss in both. ulog is my attempt at building this library. It's base API is compatible with that of debug and loglevel and with the console, making it a drop-in replacement for all of these in many cases. It has a configuration mechanism that is compatible with that of debug, but that is more powerful and is monitored for changes at runtime. It accepts configuration from the querystring allowing you to craft URLs with log config embedded in it. It has powerful, configurable formatting included by default and it does this without mangling the call stack, so the filename/line number entries in the browser console remain unharmed. You can specify where the log output should go and where it should drain. It's completely modular, so you can not only easily add features through 'mods', but you can actually even drop features you don't need by not loading the mods those features are in. It has native anylogger support, decoupling the client code from the logger. And it supports lazy loading so we can get all those great features without bloating our bundle.

I hope you will give ulog a try. If you have feedback on it, or found an issue, please let me know on the issue tracker.


ulog is very natural to use:

var log = require('anylogger')('my-module') // same as with `debug`
log('A log message')                        // same as with `debug`
log('info', 'An info message')              // not possible with `debug`
log('warn', 'A warning message')            // not possible with `debug`
log.info('Starting...')                     // same as with `loglevel` or console
log.log('Yeah!')                            // same as with console
log.error('Something went wrong', new Error('Oh no!'))
if (log.enabledFor('warn')) {

Note that in the code above, we import anylogger and not ulog. This way the client code is decoupled from the logger.

ulog inherits it's API from anylogger. If you are able to restrict yourself to the Anylogger API, your code will be framework independent and will work with any supported logging library.

Note that any logging code written for either debug, loglevel or the console should be able to do it's logging just like it did before, but now using a ulog logger instead. This backward compatibility should make migrating from any of these to ulog very easy. And because this is just the anylogger API, you should even be able to migrate back to debug or loglevel without any changes at all, by just including the right adapter in your entry point. Of course once you get used to ulog, you will never want to go back! :p


anylogger defines 6 logging levels, which correspond with the natively available logging functions on the console. ulog creates constants for these levels on all loggers:

log.ERROR // 1
log.WARN  // 2
log.INFO  // 3
log.LOG   // 4
log.DEBUG // 5
log.TRACE // 6

In addition, ulog adds constants for pseudo-levels that enable or completely disable all logging:

log.ALL   // 7
log.NONE  // 0


anylogger defines log.enabledFor and ulog implements it by checking the logger's current log level and whether it's in debug mode. Normally, you should not have to use this method, unless you are doing some expensive calculations only to write log output. In such a case you can write:

import anylogger from 'anylogger'
const log = anylogger('my-app')
if (log.enabledFor('info')) {


ulog adds a property level to each logger that is a numeric representation of the current log level.

if (log.level >= log.INFO) {
	log.info('This message will be logged')
log.level = log.WARN
log.info('This info message will NOT be logged.')
log.warn('This warning message WILL be logged.')
log.level = log.NONE
log.error('Logging is completely disabled.')

In general, code should not set the log level directly, but instead should rely on the host environment for the log level. See the section on configuring ulog.

To check the log level, enabledFor is preferred over the level property as it is within the anylogger API.

Default log level

I've found that it makes sense to have different default log levels in the browser and in Node. In Node, logging is often the only UI we have available and we (the devs/admins) are the only ones that will see that logging. In the browser, we have an alternative UI (the webpage itself), so logging will be less useful for normal users.

In Node

In Node, the log level defaults to info. This allows you to use info, warn and error when informing the user of stuff that happened.

In the browser

In the browser the log level defaults to warn. This means info messages will be excluded, but for most users these messages won't be relevant anyway.

Attention! Chromium-based browsers have their own level filter and by default, debug messages are filtered away.


When we support logging with some logging library, we add code to our application that many users don't actually need. There are techniques to remove this code from our production builds, but they sacrifice logging with it. In many scenarios we actually do want logging in the production builds. To assist support personel in diagnosing user problems for example. So it's beneficial if the minimum amount of code we need to load to support logging is small. This minimum amount of code we call the footprint. ulog keeps its footprint small by utilizing lazy loading.

Debug mode

Debug mode is a feature that ulog copied from debug and it responds to the same config option. Setting a logger to debug mode effectively means forcing it's log level to be at least debug:

import anylogger from 'anylogger'
const log = anylogger('my-app')
log('Hi!') // is output because logger is in debug mode


In ulog, logging is always sent to exactly one channel. By default, two channels exist: output, for messages of loggers that are in debug mode, or that are at an enabled level, and drain, for those messages that are filtered away.

By using a separate channel for the drain, we can override the default behavior of using noops for all log levels that are outside of the active levels. We could for example send all logging to a database and only later filter it, when we display it for example.

A channel has one or more configurable outputs that can optionally apply formatting to the message.


In ulog, where messages are going is completely configurable at runtime. You can even configure where discarded messages are going.

By default all log methods on a logger are associated with one of two channels, output and drain. To configure the outputs for these channels, two config options are available:

When the logger is created, each log method is sent either to the output channel, or to the drain channel, based on the current log level for that logger and whether that logger is in debug mode.

To configure the output for a logger, we assign the name of the output to use to the relevant logger:


This setting can include expressions to target individual loggers, just like the debug and log settings:


The value part is actually a kurly format string. The same syntax can be used here as for configuring formatting. If more than one output is specified, a multiplex function will be inserted that dispatches the logging to all specified outputs.

By default, the following outputs are included:

Output console

This actually is the native console object. Using the native console directly is what allows us to leave the call stack intact in the browser developer tools.

Output noop

This is just an object with a noop log function

Custom outputs

The default outputs are not very special, but the entire machinery is in place for you to easily add any custom outputs you could ever need. You can define additional outputs by making ulog use a mod with an outputs key:


import ulog from 'ulog'
  outputs: {
    custom: {
      log: function(){
        var args = [].slice.call(arguments)
        console.log.apply(console, args)
      info: function(){
        var args = [].slice.call(arguments)
        console.info.apply(console, args)

An output can either be an object with log, info, warn etc methods as shown above, or a kurly tag:


import ulog from 'ulog'
  outputs: {
    custom: function(ctx){
      return function(rec) {
        console[rec.level].apply(console, rec.message)

This way you can add outputs that send log messages to memory, a file, localStorage, a database etc etc.


Formatting is another feature that separates ulog from debug and loglevel. debug has formatting, but it is hardcoded and messes up the callstack and there is not much you can do about it. loglevel does not mess up the callstack, but it also has no formatting at all out of the box.

Ulog uses kurly to support advanced, configurable and customizable formatting, without mangling the callstack.

Configurable format

The actual format used is configurable easily via ulog's powerful configuration mechanism.

The default format string on Node is:

lvl name:20 message perf

This sacrifices the callstack for a colored and formatted message and having the perf measurements after the message i.s.o before it. The Node JS doesn't output any file name / line number information anyway.

On browsers, we want to spare the call stack, so there the default is:

lvl name perf

We don't include the message, but it will be appended as the last argument automatically. The result is nicely formatted messages with the file name / line number entries in the browser debug console intact.

To override the default format, just set config option log_format to the format you want.

Formats available out of the box include:

Format syntax

ulog uses the new options in kurly v2 to make tag open/close markers optional and enable nicer looking format strings. This means that lvl name perf and {lvl} {name} {perf} are equivalent. When using no open/close markers, any non-identifier symbols following the tag name are considered nested text. For example for the format string name:22, the name format will receive ':22' as ctx.text. This allows for parameterized formats, as ulog has done to support padding options.

Preserves callstack

ulog's formatting system has the unique (1) ability to do formatting while preserving the callstack. As long as only static kurly tags are used as formats, the call stack can remain unharmed.

(1) I do not know of any other logger out there that has this feature, but if you do know one, please let me know in the issue tracker

Included formats

Except for the message format, all included formats are static.

Format cr

Prints a 'carriage return line feed'

Format date

Prints the date the message was logged as yyyy/MM/dd.

Format lvl

Prints the level of the message as a single character:

  • 'x' for error messages
  • '!' for warning messages
  • 'i' for info messages
  • '-' for log messages
  • '>' for debug messages
  • '}' for trace messages

Format message

Prints the message, formatted and colored. Using this format breaks the callstack as it is dynamic.

Format name

Prints the logger name

Format perf

Prints the time difference between two invocations to the same logger, only if this difference is larger than 1ms. Produces output that looks like +62ms.

Format time

Prints the time the message was logged as hh:mm.

Fallback format

Any unrecognized tags are being interpreted by the fallback format. This just returns the field on the log record whose name matches. For example suppose we'd write level. This is an unrecognized tag so the wildcard formatter is used, which just returns the level field from the log record. If no field on the log record matches, it returns the original text unchanged, making 'Hello World!' a valid format string.

Padding options

All included formats support some padding options. For example, to pad out the logger names to 16 characters and align the text on the left, use name<16 or name:16. To align the text on the right, use name>16.

Custom formats

ulog's formatting system is easily extendable by adding kurly tags to a key formats under a mod:

import ulog from 'ulog'
import formats from 'ulog/mods/formats'
  use: [ formats ],
  formats: {
    custom: (ctx) => (rec) => (['custom'].concat(rec.message)),
    random: (ctx, rec) => () => Math.random()

These tags then become available in the format string.

We used two different signatures here, because there are two types of formats.

Types of formats

Formats come in two flavors:

Dynamic formats

Dynamic formats have full access to the message. But they do mess up the call stack. A dynamic format has this signature:

  use: [ require('ulog/mods/formats') ],
  formats: {
    dynamicFormat: function(ctx) {
      // one-time init here
      return function(rec) {
        // rec.message contains full message
        return /* ... */

Static formats

Static formats do not have access to the message. But they do not break the call stack! So prefer static formats if possible.

Static formats have this signature:

  use: [ require('ulog/mods/formats') ],
  formats: {
    staticFormat: function(ctx, rec) {
      // one-time init here
      return function(){
        // rec.message is undefined
        // rec.name, rec.level etc is populated
        return /* ... */

To read more about kurly and custom kurly tags, refer to the kurly documentation on creating kurly tags


Who doesn't like some colors?! Apart from making things prettier, when used correctly they can actually also make our logs easier to read. Now I don't know about you, but I find reading logs hard, so I'll take all the help I can get!

If you don't want colors, you can suppress them using config option log_color.


Browsers have great debug consoles these days. They even include stacktrace info for higher-level messages. But they did mess one thing up imho; the messages at these higher levels are indented a bit more than the other messages, making the logging harder to read. This can be clearly seen in the screenshot from ulog v2.0.0-beta-11, which did not yet have alignment:


ulog now automatically adds some formatting that negates the extra indentation the messages at these higher levels get, so all messages are nicely aligned:


You can control alignment with config option log_align.


ulog features a simple, yet powerful and flexible configuration mechanism. On Node JS, we can configure ulog via program arguments, environment variables or a configuration file. On browsers, we use querystring arguments or localStorage. On both platforms, the configuration is monitored and changes to it are picked up by ulog at runtime without the need to restart the application.

ulog's configuration mechanism is an extension to that of debug and is compatible with it. debug is one of the most popular logging packages in the NPM ecosystem, with tens of thousands of packages depending on it, so having ulog's configuration mechanism be compatible with it makes for a very smooth migration path. If your app or library is currently using debug, you should be able to replace it with ulog with no or only minor changes.

We configure ulog by adjusting configuration options.

  • log: The main setting to control logger's levels with
  • debug: For compatibility with debug
  • log_config: To specify the configuration file (Node JS only)
  • log_output: To configure where logging should go
  • log_drain: To configure where logs should drain
  • log_format: To configure the format for log messages
  • log_color: To enable or disable colors
  • log_align: To enable or disable alignment

Log configuration syntax

debug has a simple but powerful configuration mechanism. You set an environment variable or localStorage option named DEBUG and you assign it a value that expresses which loggers to enable. E.g.:


The format is basically a comma-separated list of logger names, using the asterisk as a wildcard character and optionally negating the expression by preceding it with a minus sign. So the expression above includes test and my:* loggers, except for my:lib which is excluded.

ulog extends this configuration mechanism. With debug, you can only turn loggers on and off, but ulog allows for a much more varied range of options. This is achieved by extending the configuration syntax so it also accepts the value applicable for the loggers matching the expression. Also we allow a semicolon-separated list of such expression=value pairs. For example, to set the logger test to debug and my:* loggers except for my:lib to info, we could write:


If an option only contains a value, ulog implicitly adds * as the expression. So we can write:


and it's equivalent to


We can even combine this. So we could write:


and it will set the level for all loggers to info, except for the ones starting with my:, which are set to debug.

A special case is the config option debug, which is designed to be compatible with debug so code using ulog will react to that setting in the same way.

Most of the config options support this syntax.

Via program arguments

On Node JS we can pass log configuration options as program arguments:

node ./myapp log=debug

This should be helpful when making CLI applications. These strongly rely on console messages, but are also often used in scripted setups where we would actually want to suppress that logging. Don't go and build in all kinds of custom methods to configure the logging but just use ulog and rely on it's powerful configuration mechanism instead.

Via environment variables

On Node JS we can pass log configuration options via environment variables:

log=debug node ./myapp

Via a config file

On Node JS we can place our log configuration in a file that will be read at startup and monitored for changes at runtime:



Via querystring parameters

In browsers, we can pass log configuration options as querystring parameters in the URL:


Via localStorage

In browsers, we can place our log configuration in localStorage and it will be read at startup and monitored for changes at runtime:

localStorage.setItem('log', 'debug')

Dynamic config

ulog's configuration mechanism watches the configuration for changes while the program is running and reacts to them in real-time. No more restarting your application just to change some log level! On Node JS, ulog watches a configuration file. Use Config option log_format to specify the location. In browsers, ulog monitors localStorage for changes.

Config option log

Configure the levels loggers should be filtered at.


Config option debug

Enables debug mode for the selected loggers.


This option is compatible with that of debug.

Config option log_config

Specify the path to the log configuration file, absolute or relative to the current working directory. Node JS only. Default to ./log.config. This option does not support expressions.


Config option log_output

Specify the name of the output logs should be written to. Defaults to 'console'.


Config option log_drain

Specify the name of the output logs should be drained to. Defaults to noop. When log messages are filtered out, they are sent to the drain instead of to the normal output. The default noop output is just a noop, but you could override this to send them to a separate file for example.


Config option log_format

Specify the format to use. Defaults to lvl name:22 message perf on Node JS and lvl name perf on browsers.

log_format=lvl name perf message;my:*=lvl name perf

This sets lvl name perf message as the default format, while assigning a different format string to all loggers starting with my:.

For more details, refer to the section on formatting

Config option log_color

Specify whether colors should be enabled. Defaults to on.


Config option log_align

Specify whether messages should be aligned. Defaults to on.



ulog is completely modular, consisting of a microscopically small core and a whole bunch of mods. The mods are small pieces of functionality, designed to be usable stand-alone or in combination with each other. All functionality that ulog adds to anylogger, it adds in the form of mods. All, except for the ability to add mods itself, in the form of method ulog.use, that is in core.

To add a mod, call ulog.use with either an individual mod

  // an empty mod

or with an array of mods:

  { /* a mod */ },
  { /* another mod */ },

A mod is an object that can define many things declaratively:

  • That it extends some core functionality
  • That it uses other mods
  • That it adds settings
  • That it adds properties to loggers
  • That it adds outputs / formats / other components
  • That it watches some config option

As an example, here is a mod that does most of these things:

var boolean = require('ulog/mods/props/boolean')

  use: [ require('./some/mod/we/use') ],
  extend: { someProp: 'addedToUlog' },
  settings: {
    cool: {
      config: 'log_cool'
      prop: boolean(),
  outputs: {
    file: {
      log: function(){
        console.info.apply(console, ['file'].concat([].slice.call(arguments)))
  formats: {
    random: function(ctx, rec){
      return function(){
        return Math.random()
  watch: {
    'debug': function(){
      console.info('debug setting changed')

Interestingly, most of these features of mods are being added by mods. For example the ability to add settings is added by the settings mod. Studying the way ulog's featureset is built from mods by reading the source code of the included mods is the best way to learn about writing mods for now.

Lazy loading

ulog being built with mods makes it easy to use lazy loading to reduce the minimum footprint. If you use ulog from a script tag (not recommended), you can get it for free. Otherwise, you have to configure your bundler to code-split.

Lazy-loading with Webpack

Have a look at ulog.bundle.js for an example of doing lazy loading with Webpack.

Basically it only loads ulog/base and then configures a watch that loads the remaining mods on-demand. It's using Webpack-specific API require.ensure for this. Other bundlers support it in similar ways.

var ulog = require('ulog/base')

// add a mod to ulog to load logging on-demand
  watch: {
    // watch for changes in these config keys
    // when changes happen, load the other mods if needed
      // webpack specific API to lazy load modules
        // ensure these modules are loaded
        [ 'ulog/mods/lazy' ],
        // then execute this function, notice require being overridden
          // use the overridden require to lazy load the modules
          if (ulog.use(require('ulog/mods/lazy'))) {
            // re-initialize the loggers if mods were added
        'ulog.lazy' // chunkname webpack will use: ulog.lazy.min.js

If you have successfully lazy-loaded ulog with other bundlers, please drop a comment in the issue tracker, or create a PR to add a section about it here in the README.

Decoupling code from debug

As I said before, debug is excellent for debug logging and there are tens of thousands of packages using it. The downside of this is that you will more or less get debug shoved through your throat if you include any of these libraries because they are tighly coupled to it. But we can uncouple them with the magic of bundlers like Webpack.

To replace debug with anylogger everywhere, you can use this Webpack alias to make all modules that use debug switch to anylogger instead:

    resolve: {
        alias: {
            'debug': 'anylogger'

This works because the anylogger API is backwards compatible with that of debug. And because ulog has native anylogger support, once you import ulog in your entry point, all those libraries that were using debug start to use ulog automagically!

Performance considerations

By default, the logging methods on the log object that correspond to a log level which is higher than the currently set level, are replaced by no-op methods. As such, you generally don't have to worry about the performance overhead of leaving the log statements in the production code. There is one exception to this rule though. If preparing the message itself is a costly operation, you may want to surround the log code with an if (log.enabledFor(level)) statement:

if (log.enabledFor('info')) {
	var message = doLotsOfWorkToGenerateLogMessage();


Add an issue in the issue tracker to let me know of any problems you find, or questions you may have.


Credits go to:

  • Felix Geisendörfer from debuggable.com for kindly giving up the ulog namespace on NPM. Thanks Felix!
  • TJ Holowaychuk for creating debug, which was a great inspiration for ulog.
  • Tim Perry for creating loglevel, which was another great inspiration for ulog.
  • Community members Jakub Jirutka and Alex Kesling for making code contributions.


Copyright 2021 by Stijn de Witt.


Licensed under the MIT Open Source license.

Package Sidebar


npm i ulog

Weekly Downloads






Unpacked Size

330 kB

Total Files


Last publish


  • felixge
  • stijndewitt