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6.3.1 • Public • Published

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Universal logging utility

Configurable, environment and presentation agnostic, with log levels and namespacing (debug style) support


1. Write application/library logs

// Default logger (writes at 'info' level)
const log = require("log");

// Log 'info' level message:
log.info("some info message %s", "injected string");

// Get namespaced logger (debug lib style)
const myLibLog = log.get("my-lib");

// Log 'info' level message in context of 'my-lib' namespace:
myLibLog.info("some info message in 'my-lib' namespace context");

// Namespaces can be further nested
const myLibFuncLog = log.get("func");

// Log 'info' level message in context of 'my-lib:func' namespace:
myLibFuncLog.info("some info message in 'my-lib:func' namespace context");

// Log 'error' level message in context of 'my-lib:func' namespace:
myLibFuncLog.error("some error message");

// log output can be dynamically enabled/disabled during runtime
const { restore } = myLibFuncLog.error.disable();
myLibFuncLog.error("error message not really logged");
// Restore previous logs visibiity state
myLibFuncLog.error("error message to be logged");

2. Initialize log writer in main (starting) process module

e.g. if target is Node.js, then install log-node, and at the top of main module initialize it


Available log levels

Mirror of applicable syslog levels (in severity order):

  • debug - debugging information (hidden by default)
  • info - a purely informational message (hidden by default)
  • notice - condition normal, but significant
  • warning (also aliased as warn) - condition warning
  • error - condition error - to notify of errors accompanied with recovery mechanism (hence reported as log and not as uncaught exception)

Note: critical, alert, emergency are not exposed as seem to not serve a use case in context of JS applications, such errors should be exposed as typical exceptions

Output message formatting

log doesn't force any specific arguments handling. Still it is recommended to assume printf-like message format, as all currently available writers are setup to support it. Placeholders support reflects one implemented in Node.js format util

Excerpt from Node.js documentation:

The first argument is a string containing zero or more placeholder tokens. Each placeholder token is replaced with the converted value from the corresponding argument. Supported placeholders are:

  • %s - String.
  • %d - Number (integer or floating point value).
  • %i - Integer.
  • %f - Floating point value.
  • %j - JSON. Replaced with the string '[Circular]' if the argument contains circular references.
  • %o - Object. A string representation of an object with generic JavaScript object formatting. Similar to util.inspect() with options { showHidden: true, depth: 4, showProxy: true }. This will show the full object including non-enumerable symbols and properties.
  • %O - Object. A string representation of an object with generic JavaScript object formatting. Similar to util.inspect() without options. This will show the full object not including non-enumerable symbols and properties.
  • %% - single percent sign ('%'). This does not consume an argument.

Note to log writer configuration developers: For cross-env compatibility it is advised to base implementation on sprintf-kit

Enabling log writing

log on its own doesn't write anything to the console or any other means (it just emits events to be consumed by preloaded log writers).

To have logs written, the pre-chosen log writer needs to be initialized in the main (starting) module of a process.

List of available log writers

Note: if some writer is missing, propose a PR

Logs Visibility

Default visibility depends on the enviroment (see chosen log writer for more information), and in most cases is setup through the following environment variables:


(defaults to notice) Lowest log level from which (upwards) all logs will be exposed.


Eventual list of namespaces to expose at levels below LOG_LEVEL threshold

List is comma separated as e.g. foo,-foo:bar (expose all foo but not foo:bar).

It follows convention configured within debug. To ease eventual migration from debug, configuration fallbacks to DEBUG env var if LOG_DEBUG is not present.

Timestamps logging

When following env var is set writers are recommended to expose timestamps aside each log message

  • rel (default) - Logs time elapsed since logger initialization
  • abs - Logs absolute time in ISO 8601 format


$ npm test

Project cross-browser compatibility supported by:

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npm i log

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