Structured Transport Browser Server Deno Bun

A console-based transport with structured logging enabled by default. Unlike the Console Transport, which requires manually configuring messageField, levelField, and dateField, the Structured Transport comes pre-configured with sensible defaults.

Transport Source

Installation

No additional packages needed beyond the core loglayer package:

npm

npm i loglayer

pnpm

pnpm add loglayer

yarn

yarn add loglayer

Setup

import { LogLayer, StructuredTransport } from 'loglayer'

const log = new LogLayer({
  transport: new StructuredTransport({
    logger: console,
  })
})

log.info('User logged in')
// console.info({ level: 'info', time: '2025-01-01T00:00:00.000Z', msg: 'User logged in' })

Default Fields

The transport outputs structured log objects with the following default fields:

Field	Default Key	Description
Level	level	The log level (e.g., info, warn, error)
Timestamp	time	ISO 8601 date string
Message	msg	The log message (multi-parameter messages are joined with a space)

Metadata and context data are merged into the log object alongside these fields.

Configuration Options

Required Parameters

None - all parameters are optional.

Optional Parameters

Name	Type	Default	Description
level	"trace" | "debug" | "info" | "warn" | "error" | "fatal"	"trace"	Sets the minimum log level to process. Messages with a lower priority level will be ignored
messageField	string	"msg"	The field name for the log message. Multi-parameter messages will be joined with a space
dateField	string	"time"	The field name for the timestamp. If dateFn is defined, will call dateFn to derive the date
levelField	string	"level"	The field name for the log level. If levelFn is defined, will call levelFn to derive the level
dateFn	() => string | number	-	If defined, a function that returns a string or number for the value to be used for the dateField
levelFn	(logLevel: LogLevelType) => string | number	-	If defined, a function that returns a string or number for a given log level
stringify	boolean	false	If true, applies JSON.stringify to the structured log output
messageFn	(params: LogLayerTransportParams) => string	-	Custom function to format the log message output. Receives log level, messages, and data; returns the formatted string

Examples

With Metadata and Context

const log = new LogLayer({
  transport: new StructuredTransport({
    logger: console,
  })
})

log.withContext({ service: 'api' })
log.withMetadata({ requestId: 'abc-123' }).info('Request received')
// console.info({
//   level: 'info',
//   time: '2025-01-01T00:00:00.000Z',
//   msg: 'Request received',
//   service: 'api',
//   requestId: 'abc-123'
// })

Custom Field Names

const log = new LogLayer({
  transport: new StructuredTransport({
    logger: console,
    messageField: 'message',
    levelField: 'severity',
    dateField: 'timestamp'
  })
})

log.info('User logged in')
// console.info({ severity: 'info', timestamp: '2025-01-01T00:00:00.000Z', message: 'User logged in' })

Custom Date Function

const log = new LogLayer({
  transport: new StructuredTransport({
    logger: console,
    dateFn: () => Date.now() // Unix timestamp instead of ISO string
  })
})

log.info('User logged in')
// console.info({ level: 'info', time: 1701437400000, msg: 'User logged in' })

Custom Level Function

const log = new LogLayer({
  transport: new StructuredTransport({
    logger: console,
    levelFn: (level) => level.toUpperCase()
  })
})

log.warn('Disk space low')
// console.warn({ level: 'WARN', time: '2025-01-01T00:00:00.000Z', msg: 'Disk space low' })

JSON String Output

const log = new LogLayer({
  transport: new StructuredTransport({
    logger: console,
    stringify: true
  })
})

log.withMetadata({ user: 'john' }).info('User logged in')
// console.info('{"level":"info","time":"2025-01-01T00:00:00.000Z","msg":"User logged in","user":"john"}')

Custom Message Formatting

import { LogLayer, StructuredTransport } from 'loglayer'
import type { LogLayerTransportParams } from 'loglayer'

const log = new LogLayer({
  transport: new StructuredTransport({
    logger: console,
    messageFn: ({ logLevel, messages }: LogLayerTransportParams) => {
      return `[${logLevel.toUpperCase()}] ${messages.join(' ')}`
    }
  })
})

log.info('User logged in')
// console.info({ level: 'info', time: '...', msg: '[INFO] User logged in' })

Prefix behavior

If you use withPrefix(), the prefix is applied to the messages before they reach messageFn. For example, log.withPrefix('[MyApp]').info('Hello') would pass messages: ['[MyApp] Hello'] to your messageFn.

Log Level Mapping

LogLayer	Console
trace	debug
debug	debug
info	info
warn	warn
error	error
fatal	error