logform
An 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
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:
{
level: 'info', // Level of the logging message
message: 'Hey! Log something?' // Descriptive message being logged.
}
logform
itself exposes several additional properties:
splat
: string interpolation splat for%d %s
-style messages.timestamp
: timestamp the message was received.label
: custom label associated with each message.
As a consumer you may add whatever properties you wish – internal state is maintained by Symbol
properties:
Symbol.for('level')
(READ-ONLY): equal tolevel
property. Is treated as immutable by all code.Symbol.for('message'):
complete string message set by "finalizing formats":json
,logstash
,printf
,prettyPrint
, andsimple
.
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 modifiedinfo
argument. Object references need not be preserved if immutability is preferred. All current built-in formats considerinfo
mutable, but [immutablejs] is being considered for future releases. - A falsey value indicating that the
info
argument should be ignored by the caller. (See: Filteringinfo
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' }
info
Objects
Filtering 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'
}))
Tests
Tests are written with mocha
, assume
, and nyc
. They can be run with npm
:
npm test