@expo/event-log is a structured event logger for command-line tools. It writes JSONL logs that can be replayed, tailed, inspected, or exported to trace formats.

In short, @expo/event-log,

• keeps logging cheap enough to leave enabled
• writes bounded session logs to the system temporary directory
• forwards worker and child-process events into the same session
• lets tools and agents tap into running commands
• derives typed event payloads through TypeScript declaration merging

Usage

Install logging once when a CLI command starts:

import { installEventLogger } from '@expo/event-log';

installEventLogger({ command: 'expo start -p web', version: '1.0.0' });

Create a logger in any package:

import { events } from '@expo/event-log';

const log = events('metro');

log('ready', { port: 8081 });

const end = log.span('bundle', { platform: 'ios' });
end('bundle', { cached: false });

Print debug-style formatted logs inline while keeping structured session logs:

LOG_DEBUG=metro:* expo start
LOG_DEBUG=* expo export

Extend EventRegistry to type event payloads:

declare module '@expo/event-log' {
  interface EventRegistry {
    'metro:ready': { port: number };
    'metro:bundle': { platform?: string; cached?: boolean };
  }
}

CLI

Use the CLI to find sessions, replay logs, or export traces:

event-log ps --json
event-log tap "expo start" --filter metro:* --tail
event-log export "expo start" --format chrome-trace -o trace.json
event-log export "expo start" --format opentelemetry -o otel.json
event-log clean --json

Selectors match a session by PID, command, session directory, or working directory. tap and export replay retained history first. --tail appends live events.

API

• events(category) creates a typed logger
• installEventLogger(options) installs logging for the process
• isEventLoggerActive() returns the active destination, or null
• list(options) lists known sessions
• tap(sessionDir, options) replays and optionally follows a session