3.1.0 • Public • Published

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(name may be subject to change)

Universal logger utility

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


Writing logs

// Default logger writes at 'debug' level
const log = require("log4");
// Log 'debug' level message:
log("some debug message %s", "injected string");
// Get namespaced logger (debug lib style)
log = log.get("my-lib");
// Log 'debug' level message in context of 'my-lib' namespace:
log("some debug message in 'my-lib' namespace context");
// Namespaces can be nested
log = log.get("func");
// Log 'debug' level message in context of 'my-lib:func' namespace:
log("some debug message in 'my-lib:func' namespace context");
// Log 'error' level message in context of 'my-lib:func' namespace:
log.error("some error message");
// log output can be dynamically enabled/disabled during runtime
const { restore } = log.error.disable();
log.error("error message not really logged");
// Restore previous logs visibiity state
log.error("error message to be logged");

Available log levels

Mirror of syslog (in severity order):

  • debug - debugging information
  • info - a purely informational message
  • notice - condition normal, but significant
  • warning (also aliased as warn) - condition warning
  • error - condition error
  • critical - condition critical
  • alert - immediate action required
  • emergency - system unusable

Output message formatting

log4 doesn't force any specific arguments handling. Still it is recommended to assume printf-like message format, as all 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 compatibiity it is advised to base implementation on sprintf-kit

Enabling log writing

log4 on its own doesn't write anything to the console on any other mean (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 main (starting) module of a process.

List of available log writers

Note: add any new writers via PR

Logs Visibility

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


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  • medikoo