logform

What is logform?

The logform npm package is a module designed for formatting messages in logging systems, primarily used with the popular Winston logger. It provides a way to define how log messages should be formatted before they are written to a console, file, or any other transport.

What are logform's main functionalities?

Simple message formatting

This feature allows users to create custom message formats using the printf function from logform. It enables the inclusion of timestamp, label, level, and message in the log output.

const { format } = require('logform');
const { printf } = format;

const myFormat = printf(({ level, message, label, timestamp }) => {
  return `${timestamp} [${label}] ${level}: ${message}`;
});

// Use this format in a Winston logger

Colorizing the output

This feature provides an easy way to add color to log messages based on the log level, enhancing readability especially when viewing logs directly from consoles.

const { format } = require('logform');
const { colorize } = format;

const colorizedFormat = colorize();
// This format can be used to add colors to different levels of log messages when used with Winston logger

Combining multiple formats

This feature allows the combination of multiple formatting methods such as adding labels, timestamps, and custom printf templates to create a highly customized log output.

const { format } = require('logform');
const { combine, timestamp, label, printf } = format;

const combinedFormats = combine(
  label({ label: 'My App' }),
  timestamp(),
  printf(info => `${info.timestamp} [${info.label}] ${info.level}: ${info.message}`)
);

// This combined format can be used in a Winston logger to provide a rich structured output

Other packages similar to logform

winston

Winston is a multi-transport async logging library for Node.js. While logform is primarily used as a formatting tool within Winston, Winston itself provides more comprehensive logging solutions including transports management, which logform does not handle.

bunyan

Bunyan is another logging library that focuses on JSON logging. Unlike logform, which is used for formatting log messages, Bunyan handles both the logging and formatting but emphasizes a JSON structure for output, making it ideal for machine parsing.

pino

Pino is a very low overhead Node.js logger, similar to Bunyan but focuses on performance. It includes its own set of formatting capabilities but is generally used for its performance benefits over extensive formatting options.

README

logform

A mutable object-based log format designed for chaining & objectMode streams.

const { format } = require('logform');

const alignedWithColorsAndTime = format.combine(
  format.colorize(),
  format.timestamp(),
  format.align(),
  format.printf(info => `${info.timestamp} ${info.level}: ${info.message}`)
);

• info Objects
• Understanding formats
  • Combining formats
  • Filtering info objects
• Formats
  • Align
  • CLI
  • Colorize
  • Combine
  • Errors
  • JSON
  • Label
  • Logstash
  • Metadata
  • PadLevels
  • PrettyPrint
  • Printf
  • Simple
  • Splat
  • Timestamp
  • Uncolorize

info Objects

The info parameter provided to a given format represents a single log message. The object itself is mutable. Every info must have at least the level and message properties:

const info = {
  level: 'info',                 // Level of the logging message
  message: 'Hey! Log something?' // Descriptive message being logged.
}

Properties besides level and message are considered as "meta". i.e.:

const { level, message, ...meta } = info;

Several of the formats in logform itself add additional properties:

Property	Format added by	Description
splat	splat()	String interpolation splat for %d %s-style messages.
timestamp	timestamp()	timestamp the message was received.
label	label()	Custom label associated with each message.
ms	ms()	Number of milliseconds since the previous log message.

As a consumer you may add whatever properties you wish – internal state is maintained by Symbol properties:

• Symbol.for('level') (READ-ONLY): equal to level property. Is treated as immutable by all code.
• Symbol.for('message'): complete string message set by "finalizing formats":
  • json
  • logstash
  • printf
  • prettyPrint
  • simple
• Symbol.for('splat'): additional string interpolation arguments. Used exclusively by splat() format.

These Symbols are stored in another package: triple-beam so that all consumers of logform can have the same Symbol reference. i.e.:

const { LEVEL, MESSAGE, SPLAT } = require('triple-beam');

console.log(LEVEL === Symbol.for('level'));
// true

console.log(MESSAGE === Symbol.for('message'));
// true

console.log(SPLAT === Symbol.for('splat'));
// true

Understanding formats

Formats are prototypal objects (i.e. class instances) that define a single method: transform(info, opts) and return the mutated info

• info: an object representing the log message.
• opts: setting specific to the current instance of the format.

They are expected to return one of two things:

• An info Object representing the modified info argument. Object references need not be preserved if immutability is preferred. All current built-in formats consider info mutable, but [immutablejs] is being considered for future releases.
• A falsey value indicating that the info argument should be ignored by the caller. (See: Filtering info Objects) below.

logform.format is designed to be as simple as possible. To define a new format simple pass it a transform(info, opts) function to get a new Format.

The named Format returned can be used to create as many copies of the given Format as desired:

const { format } = require('logform');

const volume = format((info, opts) => {
  if (opts.yell) {
    info.message = info.message.toUpperCase();
  } else if (opts.whisper) {
    info.message = info.message.toLowerCase();
  }

  return info;
});

// `volume` is now a function that returns instances of the format.
const scream = volume({ yell: true });
console.dir(scream.transform({
  level: 'info',
  message: `sorry for making you YELL in your head!`
}, scream.options));
// {
//   level: 'info'
//   message: 'SORRY FOR MAKING YOU YELL IN YOUR HEAD!'
// }

// `volume` can be used multiple times to create different formats.
const whisper = volume({ whisper: true });
console.dir(whisper.transform({
  level: 'info',
  message: `WHY ARE THEY MAKING US YELL SO MUCH!`
}), whisper.options);
// {
//   level: 'info'
//   message: 'why are they making us yell so much!'
// }

Combining formats

Any number of formats may be combined into a single format using format.combine. Since format.combine takes no opts, as a convenience it returns pre-created instance of the combined format.

const { format } = require('logform');
const { combine, timestamp, label } = format;

const labelTimestamp = combine(
  label({ label: 'right meow!' }),
  timestamp()
);

const info = labelTimestamp.transform({
  level: 'info',
  message: 'What time is the testing at?'
});

console.dir(info);
// { level: 'info',
//   message: 'What time is the testing at?',
//   label: 'right meow!',
//   timestamp: '2017-09-30T03:57:26.875Z' }

Filtering info Objects

If you wish to filter out a given info Object completely then simply return a falsey value.

const ignorePrivate = format((info, opts) => {
  if (info.private) { return false; }
  return info;
});

console.dir(ignorePrivate.transform({
  level: 'error',
  message: 'Public error to share'
}));
// { level: 'error', message: 'Public error to share' }

console.dir(ignorePrivate.transform({
  level: 'error',
  private: true,
  message: 'This is super secret - hide it.'
}));
// false

Use of format.combine will respect any falsey values return and stop evaluation of later formats in the series. For example:

const { format } = require('logform');
const { combine, timestamp, label } = format;

const willNeverThrow = format.combine(
  format(info => { return false })(), // Ignores everything
  format(info => { throw new Error('Never reached') })()
);

console.dir(willNeverThrow.transform({
  level: 'info',
  message: 'wow such testing'
}))

Formats

Align

The align format adds a \t delimiter before the message to align it in the same place.

const { format } = require('logform');

const alignFormat = format.align();

const info = alignFormat.transform({
  level: 'info',
  message: 'my message'
});

console.log(info);
// { level: 'info', message: '\tmy message' }

This was previously exposed as { align: true } in winston < 3.0.0.

CLI

The cli format is a combination of the colorize and the padLevels formats. It turns a log info object into the same format previously available in winston.cli() in winston < 3.0.0.

const { format } = require('logform');
const LEVEL = Symbol.for('level');

const cliFormat = format.cli({ colors: { info: 'blue' }});

const info = cliFormat.transform({
  [LEVEL]: 'info',
  level: 'info',
  message: 'my message'
}, { all: true });

console.log(info);
// { level: '\u001b[34minfo\u001b[39m',
//   message: '\u001b[34m    my message\u001b[39m',
//   [Symbol(level)]: 'info',
//   [Symbol(message)]:
//    '\u001b[34minfo\u001b[39m:\u001b[34m    my message\u001b[39m' }

Colorize

The colorize format adds different colors depending on the log level to the message and/or level. It accepts the following options:

• level: If set to true the color will be applied to the level.
• all: If set to true the color will be applied to the message and level.
• message: If set to true the color will be applied to the message.
• colors: An object containing the colors for the log levels. For example: { info: 'blue', error: 'red' }

const { format } = require('logform');
const LEVEL = Symbol.for('level');

const colorizeFormat = format.colorize({ colors: { info: 'blue' }});

const info = colorizeFormat.transform({
  [LEVEL]: 'info',
  level: 'info',
  message: 'my message'
}, { all: true });

console.log(info);
// { level: '\u001b[34minfo\u001b[39m',
//   message: '\u001b[34mmy message\u001b[39m',
//   [Symbol(level)]: 'info' }

This was previously exposed as { colorize: true } to transports in winston < 3.0.0.

Combine

The combine Format allows to combine multiple formats:

const { format } = require('logform');
const { combine, timestamp, json } = format;

const jsonWithTimestamp = combine(
  timestamp(),
  json()
);

const info = jsonWithTimestamp.transform({
  level: 'info',
  message: 'my message'
});

console.log(info);
// { level: 'info',
//   message: 'my message',
//   timestamp: '2018-10-02T15:03:14.230Z',
//   [Symbol(message)]:
//    '{"level":"info","message":"my message","timestamp":"2018-10-02T15:03:14.230Z"}' }

Errors

The errors format allows you to pass in an instance of a JavaScript Error directly to the logger. It allows you to specify whether not to include the stack-trace.

const { format } = require('logform');
const { errors } = format;

const errorsFormat = errors({ stack: true })

const info = errorsFormat.transform(new Error('Oh no!'));

console.log(info);
// Error: Oh no!
//     at repl:1:13
//     at ContextifyScript.Script.runInThisContext (vm.js:50:33)
//     at REPLServer.defaultEval (repl.js:240:29)
//     at bound (domain.js:301:14)
//     at REPLServer.runBound [as eval] (domain.js:314:12)
//     at REPLServer.onLine (repl.js:468:10)
//     at emitOne (events.js:121:20)
//     at REPLServer.emit (events.js:211:7)
//     at REPLServer.Interface._onLine (readline.js:282:10)
//     at REPLServer.Interface._line (readline.js:631:8)

It will also handle { message } properties as Error instances:

const { format } = require('logform');
const { errors } = format;

const errorsFormat = errors({ stack: true })

const info = errorsFormat.transform({
  message: new Error('Oh no!')
});

console.log(info);
// Error: Oh no!
//     at repl:1:13
//     at ContextifyScript.Script.runInThisContext (vm.js:50:33)
//     at REPLServer.defaultEval (repl.js:240:29)
//     at bound (domain.js:301:14)
//     at REPLServer.runBound [as eval] (domain.js:314:12)
//     at REPLServer.onLine (repl.js:468:10)
//     at emitOne (events.js:121:20)
//     at REPLServer.emit (events.js:211:7)
//     at REPLServer.Interface._onLine (readline.js:282:10)
//     at REPLServer.Interface._line (readline.js:631:8)

JSON

The json format uses safe-stable-stringify to finalize the message. It accepts the following options:

• replacer: A function that influences how the info is stringified.
• space: The number of white space used to format the json.

const { format } = require('logform');

const jsonFormat = format.json();

const info = jsonFormat.transform({
  level: 'info',
  message: 'my message',
});
console.log(info);
// { level: 'info',
//   message: 'my message',
//   [Symbol(message)]: '{"level":"info","message":"my message"}' }

This was previously exposed as { json: true } to transports in winston < 3.0.0.

Label

The label format adds the specified label before the message or adds it to the info object. It accepts the following options:

• label: A label to be added before the message.
• message: If set to true the label will be added to info.message. If set to false the label will be added as info.label.

const { format } = require('logform');

const labelFormat = format.label();

const info = labelFormat.transform({
  level: 'info',
  message: 'my message'
}, { label: 'my label', message: true });

console.log(info);
// { level: 'info', message: '[my label] my message' }

This was previously exposed as { label: 'my label' } to transports in winston < 3.0.0.

Logstash

The logstash Format turns a log info object into pure JSON with the appropriate logstash options.

const { format } = require('logform');
const { logstash, combine, timestamp } = format;

const logstashFormat = combine(
  timestamp(),
  logstash()
);

const info = logstashFormat.transform({
  level: 'info',
  message: 'my message'
});

console.log(info);
// { level: 'info',
//   [Symbol(message)]:
//    '{"@message":"my message","@timestamp":"2018-10-02T11:04:52.915Z","@fields":{"level":"info"}}' }

This was previously exposed as { logstash: true } to transports in winston < 3.0.0.

Metadata

The metadata format adds a metadata object to collect extraneous data, similar to the metadata object in winston 2.x. It accepts the following options:

• key: The name of the key used for the metadata object. Defaults to metadata.
• fillExcept: An array of keys that should not be added to the metadata object.
• fillWith: An array of keys that will be added to the metadata object.

const { format } = require('logform');

const metadataFormat = format.metadata();

const info = metadataFormat.transform({
  level: 'info',
  message: 'my message',
  meta: 42
});

console.log(info);
// { level: 'info', message: 'my message', metadata: { meta: 42 } }

PadLevels

The padLevels format pads levels to be the same length.

const { format } = require('logform');
const LEVEL = Symbol.for('level');

const padLevelsFormat = format.padLevels();

const info = padLevelsFormat.transform({
  [LEVEL]: 'info',
  message: 'my message'
});

console.log(info);
// { message: '    my message', [Symbol(level)]: 'info' }

This was previously exposed as { padLevels: true } to transports in winston < 3.0.0.

PrettyPrint

The prettyPrint format finalizes the message using util.inspect. It accepts the following options:

• depth: A number that specifies the maximum depth of the info object being stringified by util.inspect. Defaults to 2.
• colorize: Colorizes the message if set to true. Defaults to false.

The prettyPrint format should not be used in production because it may impact performance negatively and block the event loop.

NOTE: the LEVEL, MESSAGE, and SPLAT symbols are stripped from the output message by design.

This was previously exposed as { prettyPrint: true } to transports in winston < 3.0.0.

const { format } = require('logform');

const prettyPrintFormat = format.prettyPrint();

const info = prettyPrintFormat.transform({
  [LEVEL]: 'info',
  level: 'info',
  message: 'my message'
});

console.log(info);
// { level: 'info',
//   message: 'my message',
//   [Symbol(level)]: 'info',
//   [Symbol(message)]: '{ level: \'info\', message: \'my message\' }' }

Printf

The printf format allows to create a custom logging format:

const { format } = require('logform');

const myFormat = format.printf((info) => {
  return `${info.level} ${info.message}`;
})

const info = myFormat.transform({
  level: 'info',
  message: 'my message'
});

console.log(info);
// { level: 'info',
//   message: 'my message',
//   [Symbol(message)]: 'info my message' }

Simple

The simple format finalizes the info object using the format: level: message stringifiedRest.

const { format } = require('logform');
const MESSAGE = Symbol.for('message');

const simpleFormat = format.simple();

const info = simpleFormat.transform({
  level: 'info',
  message: 'my message',
  number: 123
});
console.log(info[MESSAGE]);
// info: my message {number:123}

Splat

The splat format transforms the message by using util.format to complete any info.message provided it has string interpolation tokens.

const { format } = require('logform');

const splatFormat = format.splat();

const info = splatFormat.transform({
  level: 'info',
  message: 'my message %s',
  splat: ['test']
});

console.log(info);
// { level: 'info', message: 'my message test', splat: [ 'test' ] }

Any additional splat parameters beyond those needed for the % tokens (aka "metas") are assumed to be objects. Their enumerable properties are merged into the info.

const { format } = require('logform');

const splatFormat = format.splat();

const info = splatFormat.transform({
  level: 'info',
  message: 'my message %s',
  splat: ['test', { thisIsMeta: true }]
});

console.log(info);
// { level: 'info',
//   message: 'my message test',
//   thisIsMeta: true,
//   splat: [ 'test' ] }

This was previously exposed implicitly in winston < 3.0.0.

Timestamp

The timestamp format adds a timestamp to the info. It accepts the following options:

• format: Either the format as a string accepted by the fecha module or a function that returns a formatted date. If no format is provided new Date().toISOString() will be used.
• alias: The name of an alias for the timestamp property, that will be added to the info object.

const { format } = require('logform');

const timestampFormat = format.timestamp();

const info = timestampFormat.transform({
  level: 'info',
  message: 'my message'
});

console.log(info);
// { level: 'info',
//   message: 'my message',
//   timestamp: '2018-10-02T11:47:02.682Z' }

It was previously available in winston < 3.0.0 as { timestamp: true } and { timestamp: function:String }.

Uncolorize

The uncolorize format strips colors from info objects. It accepts the following options:

• level: Disables the uncolorize format for info.level if set to false.
• message: Disables the uncolorize format for info.message if set to false.
• raw: Disables the uncolorize format for info[MESSAGE] if set to false.

This was previously exposed as { stripColors: true } to transports in winston < 3.0.0.

Tests

Tests are written with mocha, assume, and nyc. They can be run with npm:

npm test

LICENSE: MIT

AUTHOR: Charlie Robbins

Changelog

2.3.2

2022/01/09

This update, which also used up version number 2.3.1, pinned the version of the colors dependency due to vandalism by a developer of that package, as noted here. As discussed in the winston package, all users should update to this (or a later version, should one exist) ASAP.