Karhu node.js logging

Karhu is a simple logging library for node.js. The main feature that sets it apart is the ability to specify different log levels for different modules, so that you don't need to see the debug logs of the entire code base just to find the thing you are interested in.

Installation

npm install --save karhu

Usage

Basic usage

Let's start with a simple example

const log = require('karhu').context('app')
// ...
log.info('Server startup complete')
log.error(new Error('Startup failed'))

es5 compatibility

If you need es5-compatible version of the modules, you can access karhu via require('karhu/lib/karhu-es5) and the default config via require('karhu/config/default-e5'). You might not have as much success as you'd like though, as the ansi-styles dependency relies on arrow functions at the very least.

Still, if you stub that out then you can get karhu running in IE11.

API

For all the code examples presented here, const karhu = require('karhu') is expected to be present in the file before the examples.

Creating a logger

By default a karhu logger is set up with the default build in configuration. Your application can set up a different configuration to be used.

Changing the default config (which should be used by libraries using karhu) can be done by

karhu.configure(newConfig) 

where newConfig of course is the new configuration.

You can also create karhu loggers with specific configuration objects without changing the default by calling

const configuredKarhu = karhu.usingConfig(newConfig)

You can then use configuredKarhu as if it was karhu itself, is just uses a different set of configuration options.

Every karhu logger has context. This allows for making events from different sources distinctly different. The context is a string; you can specify whatever you want.

const log = karhu.context('mymodulename')

Updating configuration

If you want to update configuration, you can call reconfigure on any karhu instance, including the default one. All existing contexts using that instance will use the new configuration from then on.

Log levels cannot be updated with this API at this time.

karhu.reconfigure({defaultLogLevel: 'DEBUG'})

Wrapping output outside of explicit logging

You may want to enforce a consistent format to all output, even if it is done by, let's say a library that uses console.log.

This is something karhu can help you with. By calling captureStandardOutput you set up capturing of standard output and turning that into log events.

By default everything to stdout becomes INFO events and everything to stderr becomes ERROR events.

The function must be provided with the context it will use.

karhu.captureStandardOutput(karhu.context('captured-output'), 'INFO', 'ERROR')

If you for whatever reason want to stop capturing the output, you can call abort() on the value returned by the call

const abortable = karhu.captureStandardOutput(karhu.context('captured-output'))
abortable.abort()

Logging

A logger has methods for all of the specified logging levels. The default levels are

• none; never to be printed; not useful for logging, but can be used to disable logging
• debug
• info
• warn
• error

Each method can take any number of all kinds of arguments and they will (by default) be printed just as console.log would.

Configuration

Karhu is configured using a javascript object, that has properties presented below. You probably want to base your config based on the default config, which can be obtained using require('karhu/config/default')

logLevels

logLevels is an array of strings, which are the log levels available for logging. The log levels in this array should generally be in ALL CAPS, they will be available in the logger both using the presented form as well as in all lowercase. In other words, ERROR here allows both log.error and log.ERROR to be used.

If multiple log levels map to the same lowercase form (and neither is all lowercase), then the first one encountered will be made available.

The log levels must be presented in ascending order of importance.

const logLevels = ['INF', 'WARN', 'ERR']

colors

Karhu supports colors to emphasize more important information. colors is an object that maps log levels to their colors. Colors are expected to be colors from the ansi-styles node.js library, or compatible.

A single color can be defined, or an array of colors if you want to change both the foreground and background colors, for example.

const colors = {
    ERROR: [styles.color.whiteBright, styles.bgColor.redBright],
    NOTICE: styles.color.blueBright
}

You can also specify color for default, which is used if no other color matches.

outputFormat

The format that is to be used for output. The default config includes support for text and json, but you can specify additional formats in formatters. The default config defaults to text, unless the environment variable KARHU_JSON is set to a truthy value.

formatters

Formatters transform the logged data into something that can be output. Typically that means strings, but anything accepted by the transports of the config can be used.

The default config includes formatters for text and json.

formatNow

This option should contain a function that returns the current moment timestamp in whatever format is desired for the log.

The default config uses new Date().toISOString(), which produces a timestamp that looks pretty much like this: 2018-02-08T11:20:25.690Z, always in UTC.

The default formatters utilize this function, but custom formatters might not.

const formatNow = config => new Date().toISOString()

The function gets the active configuration as a parameter, in case you want to make the decision based on output format, for example.

contextSpecificLogLevels

This map or object allows you to specify log levels for individual contexts to be different from the default.

The context can be either the specific context as a string or a regular expression that will apply the log level to any matching context. Specific log level on a context always overrides regular expressions.

const contextSpecificLogLevels = new Map([
    ["src/app", "ERROR"], // ignore everything less than an error
    [/special/, "DEBUG"]
])

const contextSpecificLogLevels = {
    "src/app": "ERROR"
}

Regular expressions are not supported when the object form is used.

defaultLogLevel

This string specifies the default log level -- anything lower in importance will not be logged.

envVariablePrefix

This variable can be used to specify a prefix for environment variables used for runtime configuration. By default the prefix is just KARHU.

outputMapper

This is a function that maps data being output to another form. It is given a value being logged along with other relevant information and it is expected to return whatever is to be passed to console.log (or one of its siblings).

const outputMapper = (value, logLevel, context, toLog) => {
    if (value === undefined) return '(undefined)'
    return value    
} 

transports

Transports is an object hash from transport name to means of generating the output. A transport can override most of the settings from the top level config, inheriting everything not overridden.

const transports = {
    console: consoleTransport     
}

The default implementation includes console transport, which calls console.error for errors, console.warn for warning and console.log for everything else.

The sections below describe the transport options not available on top level config

supportsColor

A boolean or function that is to return a boolean set to true if (ANSI) colors are supported by the transport.

const supportsColor = () => false

outputImpl

const consoleTransport = {
     ERROR: (toLog: any[]) => console.error(toLog),
     WARN: (toLog: any[]) => console.warn(toLog), 
     default: (toLog: any[]) => console.log(toLog)
])

const outputImpl = {console: consoleTransport}

The functions receive the things being logged as an array (after applying mapping of outputMapper), the log level and context for the event and the active configuration.

You can specify a different methdod for each log level, and optionally a default which will be used whenever there is no function specified for the one being used.

Run-time configuration

You can override some configuration options using environment variables.

Note that the configuration can change the KARHU prefix of the environment variables to become something else.

Overriding default log level

You can set a different default log level to be used using the KARHU_LOG_LEVEL environment variable.

KARHU_LOG_LEVEL=debug node src/app

Overriding context-specific log level

You can set a different log level for an individual module using environment variables that include the context.

KARHU_LOG_LEVEL_src_app=DEBUG node src/app # set DEBUG as the log level for src/app

Any sequences for non-English-alphanumeric characters in the module name are replaced by underscores in the environment variable name.

Development

Running tests

npm test

Releasing

npm release patch # +0.+0.+1
npm release minor # +0.+1.0
npm release major # +1.0.0