Getting Started

Quick Start

Get up and running with evlog in minutes. Learn the log API, createLogger for wide events, useLogger for requests, and structured errors.

This guide covers the core APIs you'll use most often with evlog.

In Nuxt, evlog auto-imports all functions (useLogger, log, createError, parseError). No import statements needed.

Get evlog running in 2 minutes

log (Simple Logging)

The simplest way to use evlog. Fire-and-forget structured logs, anywhere in your code:

import { log } from 'evlog'

log.info('auth', 'User logged in')
log.error({ action: 'payment', error: 'card_declined' })
log.warn('cache', 'Cache miss')

10:23:45.612 [auth] User logged in
10:23:45.613 ERROR [my-app] action=payment error=card_declined
10:23:45.614 [cache] Cache miss

Two call styles:

Tagged: log.info('tag', 'message') for quick, readable console output
Structured: log.info({ key: value }) for rich events that flow through the drain pipeline

See the full Simple Logging guide for all patterns and drain integration.

createLogger (Wide Events)

When you need to accumulate context across multiple steps of an operation, whether a script, background job, queue worker, or workflow, use createLogger:

import { initLogger, createLogger } from 'evlog'

initLogger({ env: { service: 'sync-worker' } })

const log = createLogger({ jobId: job.id, queue: 'emails' })

log.set({ batch: { size: 50 } })
log.set({ batch: { processed: 50 } })
log.emit()

10:23:45.612 INFO [sync-worker] in 1204ms
  ├─ jobId: job_abc123
  ├─ queue: emails
  └─ batch: size=50 processed=50

createLogger() accepts any initial context as a plain object. It returns a logger with set, error, info, warn, emit, and getContext.

For HTTP request contexts specifically, use createRequestLogger() which pre-populates method, path, and requestId:

src/worker.ts

import { createRequestLogger } from 'evlog'

const log = createRequestLogger({ method: 'POST', path: '/api/checkout' })

With createLogger and createRequestLogger, you must call log.emit() manually. In framework integrations, this happens automatically.

useLogger (Retrieve the Request Logger)

When using a framework integration (Nuxt, Hono, Express, etc.), the middleware automatically creates a wide event logger on request start and emits it on response end. useLogger(event) retrieves that logger from the request context:

import { useLogger } from 'evlog'

export default defineEventHandler(async (event) => {
  const log = useLogger(event)

  log.set({ user: { id: 1, plan: 'pro' } })
  log.set({ cart: { items: 3, total: 9999 } })

  const order = await processCheckout()
  log.set({ orderId: order.id })

  return { success: true, orderId: order.id }
})

10:23:45.612 INFO [my-app] POST /api/checkout 200 in 234ms
  ├─ user: id=1 plan=pro
  ├─ cart: items=3 total=9999
  └─ orderId: ord_abc123

useLogger doesn't create a logger, the framework middleware already did that. It just retrieves it from the event context so you can add data with set().

When to use what

Use `log`	Use `createLogger()` / `createRequestLogger()`	Use `useLogger(event)`
Quick one-off events	Scripts, jobs, workers, queues, HTTP without a framework	API routes with a framework integration
No context accumulation needed	Accumulate context over an operation	Retrieve the request-scoped logger
Client-side logging	Wide events (one log per operation)	Access the auto-managed wide event

Service Identification

In multi-service architectures, differentiate which service a log belongs to using either route-based configuration or explicit service names.

Route-Based Configuration

Configure service names per route pattern in your nuxt.config.ts:

nuxt.config.ts

export default defineNuxtConfig({
  modules: ['evlog/nuxt'],

  evlog: {
    env: {
      service: 'default-service',
    },
    routes: {
      '/api/auth/**': { service: 'auth-service' },
      '/api/payment/**': { service: 'payment-service' },
      '/api/booking/**': { service: 'booking-service' },
    },
  },
})

Logs from routes matching these patterns will automatically include the configured service name:

Output

21:57:10.442 INFO [auth-service] POST /api/auth/login 200 in 1ms
  ├─ requestId: 88ced16a-bef2-4483-86cb-2b4fb677ea52
  ├─ user: id=user_123 email=demo@example.com
  └─ action: login

Explicit Service Parameter

Override the service name for specific routes using the second parameter of useLogger:

server/api/legacy/process.post.ts

import { useLogger } from 'evlog'

export default defineEventHandler((event) => {
  const log = useLogger(event, 'legacy-service')

  log.set({ action: 'process_legacy_request' })

  return { success: true }
})

Priority order: Explicit useLogger parameter > Route configuration > env.service > Auto-detected from environment

createError (Structured Errors)

Use createError() to throw errors with actionable context:

import { createError } from 'evlog'

throw createError({
  message: 'Payment failed',
  status: 402,
  why: 'Card declined by issuer',
  fix: 'Try a different payment method',
  link: 'https://docs.example.com/payments/declined',
})

{
  "statusCode": 402,
  "message": "Payment failed",
  "data": {
    "why": "Card declined by issuer",
    "fix": "Try a different payment method",
    "link": "https://docs.example.com/payments/declined"
  }
}

Error Fields

Field	Required	Description
`message`	Yes	What happened (user-facing)
`status`	No	HTTP status code (default: 500)
`why`	No	Technical reason (for debugging)
`fix`	No	Actionable solution
`link`	No	Documentation URL for more info
`cause`	No	Original error (if wrapping)
`internal`	No	Backend-only fields for logs and wide events — never included in HTTP JSON or `parseError()`

Frontend Integration

Use parseError() to extract all error fields on the client:

composables/useCheckout.ts

import { parseError } from 'evlog'

export async function checkout(cart: Cart) {
  try {
    await $fetch('/api/checkout', { method: 'POST', body: cart })
  } catch (err) {
    const error = parseError(err)

    toast.add({
      title: error.message,
      description: error.why,
      color: 'error',
      actions: error.link
        ? [{ label: 'Learn more', onClick: () => window.open(error.link) }]
        : undefined,
    })

    if (error.fix) {
      console.info(`Fix: ${error.fix}`)
    }
  }
}

log (Client-Side)

The same log API works on the client side, outputting to the browser console:

<script setup lang="ts">
async function handleCheckout() {
  log.info('checkout', 'User initiated checkout')

  try {
    await $fetch('/api/checkout', { method: 'POST' })
    log.info({ action: 'checkout', status: 'success' })
  } catch (err) {
    log.error({ action: 'checkout', error: 'failed' })
  }
}
</script>

export function useAnalytics() {
  function trackEvent(event: string, data?: Record<string, unknown>) {
    log.info('analytics', `Event: ${event}`)
    if (data) {
      log.debug({ event, ...data })
    }
  }

  return { trackEvent }
}

See Client Logging for transport configuration, identity context, and browser drain setup.

Next Steps

Logging Overview: Understand all three logging modes
Wide Events: Learn how to design effective wide events
Typed Fields: Add compile-time type safety to your wide events
Structured Errors: Master error handling with evlog
Best Practices: Security guidelines and production tips

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Installation

Install evlog in your TypeScript project. Supports Nuxt, Next.js, SvelteKit, Hono, Express, Fastify, Elysia, NestJS, and standalone scripts.

Agent Skills

AI-assisted code review and evlog adoption using Agent Skills. Let AI review your logging patterns and guide migration to wide events.