Log that in browser or Node.js

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Log That

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Logging library for browsers and Node.js.

This is a reference implementation of the logging API defined by @proc7ts/logger package.


Default logger:

import { logZ } from '@run-z/log-z';

const logger = logZ();  // Create default logger to `console`.

logger.info('Message'); // Logs with `console.info()`.
logger.error('Error');  // Logs with `console.error()`.
logger.debug('Debug');  // Messages below "Warning" discarded by default.

Logger that logs all messages, including debug and trace.

import { logZ, zlogDetails } from '@run-z/log-z';

// Create default logger to `console` that logs all messages.
const logger = logZ({ atLeast: 0 });

logger.debug('Debug');  // Logs with `console.log()`.
logger.trace('Without stack trace'); // Logs with `console.debug()`.
logger.trace('With stack trace', zlogDetails({ stackTrace: true })); // Logs with `console.trace()`.

Node.js Support

Log to file

import { logZ, zlogDEBUG } from '@run-z/log-z';
import { logZToFile } from '@run-z/log-z/node';

// Create a file logger to `test.log`.
const fileLogger = logZ({ atLeast: zlogDEBUG, by: logZToFile('test.log') });

// [DEBUG] Message

// Create a daily rolling file logger.
const rollingLogger = logZ({
  by: logZToFile(() => {
    const now = new Date();

    return `test.${now.getFullYear()}-${now.getMonth() + 1}-${now.getDate()}.log`;

Log to arbitrary stream

import { logZ, zlogDEBUG } from '@run-z/log-z';
import { logZToStream } from '@run-z/log-z/node';

// Create a stdout logger.
const streamLogger = logZ({ atLeast: zlogDEBUG, by: logZToStream(process.stdout) });

// [INFO ] Message

Log Messages

A log message is an object with the following properties:

  • level - numeric log level. A ZLogLevel enum can be used to specify one of predefined levels:

    • Fatal - The service/app is going to stop or become unusable now. An operator should definitely look into this soon.
    • Error - Fatal for a particular request, but the service/app continues servicing other requests. An operator should look at this soon(ish).
    • Warning - A note on something that should probably be looked at by an operator eventually.
    • Info - Detail on regular operation.
    • Debug - Anything else, i.e. too verbose to be included in "info" level.
    • Trace - Logging from external libraries used by your app or very detailed application logging.
  • line - Log line containing arbitrary values to log.

  • details - Message details map.

    The keys of this map are specific to application or log recorder implementation.

The record() method of the logger logs such messages.

The logger also has more convenient methods accepting arbitrary parameters and corresponding to each predefined log level, like info() and error(), warn(), trace(), etc. Each of these methods processes Loggable values. E.g. the one created by zlogDetails().

A zlogMessage() function can be used to construct a log message. A level-specific messages can be constructed by dedicated functions like zlogDEBUG or zlogERROR.

Loggable Values

A message error or any message parameter can customize how it is logged by implementing Loggable or ZLoggable interface. I.e. it should implement toLog() method. See the @proc7ts/logger for the details.

For example, a zlogDetails() creates a loggable value that adjusts the details of the logged message.

Another possible usage scenario is deferring the actual evaluation of the logged message until it is written to the log. This can be done with logDefer() function:

import { logDefer } from '@proc7ts/logger';

logger.debug('Debug info', logDefer(() => zlogDetails({ info: evaluateDebugInfo() })));
// `evaluateDebugInfo()` will be called only if `DEBUG` log level enabled.
// Note that this will happen right before writing to the log,
// which may happen at a later time, or not happen at all.

Log Formats

The Node.js loggers write messages in predefined format by default:

import { logZ } from '@run-z/log-z';
import { logZToStream } from '@run-z/log-z/node';

const logger = logZ({ by: logZToStream(process.stdout) });

logger.info('Message', new Error('Failed'), zlogDetails({ detail1: 'value', detail2: 2 }), 'Extra 1', [2, 3])
// [INFO ] Message ("Extra 1", [2, 3]) { detail1: "value"; detail2: 2 } Error: Failed
//    at file:///.../example.mjs:6:24
//    at ModuleJob.run (internal/modules/esm/module_job.js:146:37)
//    at async Loader.import (internal/modules/esm/loader.js:182:24)
//    at async Object.loadESM (internal/process/esm_loader.js:84:5)

This format can be customized:

import { levelZLogField, logZ, messageZLogField, zlogDetails, zlogLevelName } from '@run-z/log-z';
import { logZToStream } from '@run-z/log-z/node';

const logger = logZ({
  by: logZToStream(
      format: {
        fields: [
          '>>> ',
          ' "',
          '" ',
          line => {

logger.info('Message', zlogDetails({ detail1: 'value', detail2: 2 }));
// >>> Info "Message" {"detail1":"value","detail2":2}

Or completely replaced:

import { logZ, zlogDetails } from '@run-z/log-z';
import { logZToStream } from '@run-z/log-z/node';

const logger = logZ({
  by: logZToStream(
      format: JSON.stringify,

logger.info('Message', zlogDetails({ detail1: 'value', detail2: 2 }));
// {"level":30,"text":"Message","details":{"detail1":"value","detail2":2},"extra":[]}

Custom Loggers

Custom logger can be built by implementing a ZLogRecorder interface. The latter has three methods:

  • record(message: ZLogMessage): void - Records a log message.

    The actual logging of the message can be asynchronous.

  • whenLogged(which?: 'all' | 'last'): Promise<boolean> - Awaits for the recorded message(s) to be either logged or discarded.

    The optional which parameter can be one of:

    • "all" to wait for all messages,
    • "last" (the default) to wait for the last recorded message only.
  • end(): Promise<void> - Ends log recording and returns a promise resolved when recorder stopped.

    All messages discarded after this method call.

The logZ and logZBy functions converts arbitrary ZLogRecorder to logger. The former accepts additional parameters, e.g. is able to filter out some debug messages.

Additional Log Recorders

There are several additional log recorders that can combine or modify the behavior of other recorders:

  • logZAtopOf - Logs messages atop of the target log recorder. When it ends logging, the target recorder still can record messages. This, combined with logZWithDetails, can be treated as "child" logger.
  • logZByAll - Logs messages by all the given recorders.
  • logZByAny - Logs messages by any of the given recorders.
  • logZTimestamp - Timestamp log recorder. Adds a timestamp property to message details.
  • logZToBuffer - A log buffer that can be drained to another log recorder.
  • logZToLogger - Logs to another Logger. To consoleLogger by default. This is the default recorder used by logZ function.
  • logZToOther - Logs to another log recorder provided by function.
  • logZUpdated - Updates log messages.
  • logZWhenLevel - A log recorder of messages with required level. Messages not satisfying the condition either logged by another recorder, or discarded.
  • logZWithDetails - Updates message details.