Logging

Prerequisites: npm install jaypie (includes @jaypie/logger)

Overview

Jaypie provides structured logging with request-scoped trace IDs. Logs are JSON-formatted for Datadog ingestion. Follow the log level conventions to maintain clean, actionable logs.

Quick Reference

Level	Use Case	Example
trace	Happy path checkpoints	Entering function, completing step
debug	Unexpected but handled cases	Cache miss, retry attempt
info	Significant events (rare)	Server started, migration complete
warn	Concerning but recoverable	Deprecation, approaching limit
error	Failures requiring attention	External service down
fatal	Application cannot continue	Missing critical config

Basic Usage

import { log } from "jaypie";

log.trace("Starting request processing");
log.debug("Cache miss, fetching from database");
log.info("Server started on port 3000");
log.warn("API rate limit at 80%");
log.error("External payment service unavailable");
log.fatal("Missing required MONGODB_URI");

Variable Logging

Use log.var() for structured key-value logging:

// Single key-value pair (preferred)
log.var({ userId: "abc-123" });

// Output:
// { "status": "trace", "message": "abc-123", "var": "userId", "data": "abc-123", "dataType": "string" }

Variable logging is available at all levels:

log.trace.var({ requestId: "req-123" });
log.debug.var({ cacheHit: false });
log.error.var({ errorCode: "E001" });

Variable Logging Rules

1. Single key per call - Log one variable at a time:

// Good
log.var({ userId: "123" });
log.var({ orderId: "456" });

// Bad - multiple keys
log.var({ userId: "123", orderId: "456" });

2. Don’t log large structures - Log summaries instead:

// Good
log.var({ userCount: users.length });

// Bad - large array
log.var({ users: users });

3. Separate strings from variables:

// Good
log.trace("[processOrder] starting");
log.var({ orderId: order.id });

// Bad - mixing string and variable
log.trace(`Processing order ${order.id}`);

Log Level Guidelines

Trace (Happy Path)

Use for normal execution flow. Should be the only level in successful requests:

async function getUser(id) {
  log.trace("[getUser] fetching user");
  const user = await db.users.findById(id);
  log.trace("[getUser] user found");
  return user;
}

Debug (Off Happy Path)

Use when something unexpected happens but is handled:

async function getCachedUser(id) {
  const cached = cache.get(id);
  if (!cached) {
    log.debug("[getCachedUser] cache miss");
    return await fetchAndCacheUser(id);
  }
  return cached;
}

Info (Rarely Used)

Reserve for significant application events:

// Server lifecycle
log.info("Server started");

// Major state changes
log.info("Database migration complete");

Warn (Concerning Situations)

Use for recoverable issues that need attention:

if (apiCallsThisHour > limit * 0.8) {
  log.warn("Approaching API rate limit");
  log.var({ usage: apiCallsThisHour });
}

Error (Failures)

Use for failures that need investigation:

try {
  await externalService.call();
} catch (error) {
  log.error("External service call failed");
  log.var({ service: "payment" });
  throw BadGatewayError();
}

Fatal (Application Cannot Continue)

Use when the application must stop:

if (!process.env.MONGODB_URI) {
  log.fatal("Missing required MONGODB_URI");
  process.exit(1);
}

Request Tagging

Tag all subsequent logs with context:

import { log } from "jaypie";

// Tag logs for this request
log.tag({ requestId: "req-abc-123" });
log.tag({ userId: "user-456" });

// All subsequent logs include these tags
log.trace("Processing request");
// Output includes: { requestId: "req-abc-123", userId: "user-456", ... }

Child Loggers

Create loggers with additional context:

const userLog = log.with({ userId: user.id });

userLog.trace("User action started");
userLog.trace("User action completed");
// Both logs include userId

Lambda Initialization

Reset logger state between Lambda invocations:

import { lambdaHandler, log } from "jaypie";

export const handler = lambdaHandler(async (event) => {
  // log.init() called automatically by handler
  log.trace("Processing event");
});

For manual initialization:

log.init(); // Clears tags, resets state

Session Management

Handlers automatically call log.setup() and log.teardown() to bookend each request. On teardown, a report is emitted as log.info.var({ report }) containing accumulated data and warn/error counts.

// Manual session (handlers do this automatically)
log.setup({ handler: "myHandler", invoke: "abc-123" });

// Accumulate report data during the request
log.report({ userId: "456" });
log.report({ itemCount: 3 });

// Teardown emits the report with warn/error stats
log.teardown();
// => { report: { userId: "456", itemCount: 3, log: { warn: false, warns: 0, error: false, errors: 0 } } }

log.report() only accumulates data while a session is active (after setup(), before teardown()); calling it outside a session logs a warning and is a no-op. Warn and error calls made during the session are counted automatically and included in the final report.

log.report() warns when a key is written twice. Use log.tally() for data written repeatedly — keys combine instead: numbers sum, strings collect into an array of strings, booleans AND, and objects merge recursively. Outside an active session log.tally() silently no-ops, so libraries can tally unconditionally.

log.tally({ llm: { operates: 1, turns: 2 } });
log.tally({ llm: { operates: 1, turns: 3 } });
// => teardown report includes { llm: { operates: 2, turns: 5 } }

Inside a handler, @jaypie/llm tallies an llm key automatically — operate() and stream() report turns, tool calls, and usage by model with no code changes.

Environment Configuration

Variable	Values	Default
LOG_LEVEL	trace, debug, info, warn, error, fatal	info
LOG_FORMAT	json, text	json
LOG_MAX_ENTRY_BYTES	bytes; false disables	262144 (256KB)
LOG_MAX_STRING	characters; false disables	off
LOG_MAX_DEPTH	levels; false disables	off

LOG_LEVEL=debug npm start

Oversized entries truncate deliberately with visible markers (… [truncated 612,340 chars]) so payloads fit the log pipeline — see Serialization Limits for details and log.config() runtime overrides.

Function Prefix Convention

Use bracketed function names in log messages:

async function processOrder(orderId) {
  log.trace("[processOrder] starting");
  log.var({ orderId });

  const order = await fetchOrder(orderId);
  log.trace("[processOrder] order fetched");

  await validateOrder(order);
  log.trace("[processOrder] order validated");

  return order;
}

Testing Logs

Use @jaypie/testkit for log assertions:

import { log } from "jaypie";
import { spyLog, restoreLog } from "@jaypie/testkit";

beforeEach(() => {
  spyLog(log);
});

afterEach(() => {
  restoreLog(log);
});

it("logs at trace level on happy path", async () => {
  await processOrder("order-123");

  // Verify no logs above trace
  expect(log.debug).not.toHaveBeenCalled();
  expect(log.warn).not.toHaveBeenCalled();
  expect(log.error).not.toHaveBeenCalled();
});

it("logs error on failure", async () => {
  await expect(processOrder("invalid")).rejects.toThrow();
  expect(log.error).toHaveBeenCalled();
});

Related

• Error Handling - Error types and when to log errors
• Handler Lifecycle - Automatic log initialization
• @jaypie/logger - Full API reference
• Testing - Testing log output