Getting Started

LoggerJS is an isomorphic structured logging SDK. The same core API runs in Node, browsers, workers, and edge runtimes; platform packages add transports and automatic collection on top.

Install

Pick the package for your platform. Each platform package re-exports the @loggerjs/core kernel, so one install is enough to start. Every LoggerJS package takes @loggerjs/core as a peer dependency so an application has exactly one copy of it; install it alongside, as below (npm and pnpm add a missing peer automatically, Yarn does not).

# Node services
pnpm add @loggerjs/core @loggerjs/node @loggerjs/processors

# Browser apps
pnpm add @loggerjs/core @loggerjs/browser @loggerjs/processors

All packages ship ESM and CJS entry points with full TypeScript declarations. For Node consumers, packed packages are smoke-tested on Node 22.0.0, the latest Node 22, and Node 24; Node 22 is the floor. Repository development uses Node >=22.13.0 for the full toolchain. Browser code targets Chrome and Edge 103, Firefox 100, and Safari 16 or later (see the support commitments in GOVERNANCE).

First Logger (Node)

import { captureProcessIntegration, createLogger, stdoutTransport } from "@loggerjs/node";
import { redactProcessor } from "@loggerjs/processors";

const logger = createLogger({
  category: ["api"],
  level: "info",
  tags: { service: "checkout", env: process.env.NODE_ENV ?? "dev" },
  processors: [redactProcessor()],
  transports: [stdoutTransport()],
  integrations: [captureProcessIntegration()],
});

logger.info("order created", { orderId: "ord_123" });
logger.error("payment failed", new Error("card declined"));

await logger.flush();

stdoutTransport() writes one NDJSON line per log. captureProcessIntegration() turns uncaught exceptions, unhandled rejections, and process warnings into log events automatically.

First Logger (Browser)

import {
  browserHttpTransport,
  captureBrowserErrorsIntegration,
  captureConsoleIntegration,
  createLogger,
  memoryBrowserHttpOfflineQueue,
  pageLifecycleIntegration,
} from "@loggerjs/browser";

const logger = createLogger({
  category: ["web"],
  level: "info",
  transports: [
    browserHttpTransport({
      url: "/api/logs",
      offlineQueue: memoryBrowserHttpOfflineQueue({ maxEntries: 500 }),
      useBeaconOnPageHide: true,
    }),
  ],
  integrations: [
    captureConsoleIntegration({ levels: ["warn", "error"] }),
    captureBrowserErrorsIntegration(),
    pageLifecycleIntegration(),
  ],
});

logger.info("page loaded");

The HTTP transport batches logs, queues them while offline, replays on online, and falls back to navigator.sendBeacon when the page is closing.

Levels

Six enabled levels plus silent:

Name	Value
trace	10
debug	20
info	30
warn	40
error	50
fatal	60

logger.setLevel("debug");
logger.isLevelEnabled("trace"); // false

Disabled levels cost one numeric comparison — no allocation, no context lookup, no message formatting.

Lazy Messages

Pass a function when building the message is expensive. It is only called if the level is enabled, and at most once:

logger.debug(() => `cart state: ${JSON.stringify(cart)}`);

Errors

An Error as the first argument becomes the record's error, with an optional explicit message:

logger.error(err);
logger.error(err, "payment failed", { orderId: "ord_123" });

Errors are normalized (name, message, truncated stack, enumerable properties, cause chain) before transports see them.

Child Loggers and Tags

const checkoutLogger = logger.child({
  category: ["api", "checkout"],
  tags: { domain: "checkout" },
});

Children inherit level, tags, bindings, middleware, processors, and transports; integrations are not inherited. withTags() and withType() are shorthands for common child shapes.

Ambient Context

Bind request-scoped values once instead of threading them through every call:

import { withContext } from "@loggerjs/core";
import { installAsyncLocalStorageContext } from "@loggerjs/node";

installAsyncLocalStorageContext(); // once at startup

await withContext({ requestId: "req_123" }, async () => {
  logger.info("request started"); // context: { requestId: "req_123" }
});

In the browser the default stack-based context manager covers synchronous scopes; in Node, AsyncLocalStorage carries context across await boundaries.

Typed Events

Define reusable, typed event shapes:

import { defineEvent } from "@loggerjs/core";

const CheckoutCompleted = defineEvent<{ orderId: string; amountCents: number }>({
  type: "checkout.completed",
  message: (event) => `checkout completed ${event.orderId}`,
  tags: { domain: "checkout" },
});

logger.event(CheckoutCompleted, { orderId: "ord_123", amountCents: 4999 });

Library Authors: the Registry

Libraries should not construct loggers; they look one up by category and stay silent until the host application configures output:

// In the library
import { getLogger } from "@loggerjs/core";
const logger = getLogger(["my-lib", "client"]);
logger.debug("handshake started"); // no-op until configured

// In the application
import { configure } from "@loggerjs/core";
await configure({
  transports: { stdout: stdoutTransport() },
  loggers: [{ category: ["my-lib"], level: "warn", transports: ["stdout"] }],
});

Shutdown

await logger.flush(); // drain pending transport work
await logger.close(); // tear down integrations, close transports

For crash paths, transports that support it expose flushSync(); see OPERATIONS.md.

Next Steps

• CONCEPTS.md — the pipeline model: records, events, middleware, processors, transports, codecs.
• TRANSPORTS.md — every built-in transport and how to write your own.
• PRETTY.md — browser DevTools and Node terminal pretty output.
• INTEGRATIONS.md — automatic collection for browser and Node.
• PROCESSORS.md — the middleware/processor toolbox.
• CODECS.md — serialization ownership and the codec contract.
• PERFORMANCE.md — configuring for throughput.
• OPERATIONS.md — privacy, offline queues, crash paths.
• PRODUCTION-RECIPES.md — browser HTTP/offline, Node stdout+OTLP, Loki/Datadog deployments.
• API-STABILITY.md — stable, compatible, and experimental tiers and the SemVer promise.
• MIGRATION.md — coming from pino, winston, or console.