@hyperflake/slog

A lightweight, structured logger for Node.js that mimics the text output format of Go's log/slog packages. It prints a timestamp, level, message, and key/value fields on a single line, and renders the full stack trace beneath the line whenever an Error is logged.

Features

• slog-style output: 2026/06/22 05:45:47 INFO image resized width=1920 height=1080
• JSON output: switch a logger to emit one JSON object per line instead, e.g. for log pipelines that expect structured input.
• Structured fields: pass a plain object of key/value pairs alongside any log call.
• Full error logging: errors render their message and full stack trace on a single line (escaped in logfmt, structured in JSON) — one log line in, one in your shipper.
• Child loggers: attach persistent fields to a logger with with() without mutating the original.
• Level filtering: debug, info, warn, error, each gated by a configurable minimum level.

Installation

Install the package using npm:

npm install @hyperflake/slog

Usage

Basic Setup

Use the Default Logger

import { logger } from '@hyperflake/slog';

logger.info('image resized', { width: 1920, height: 1080 });
// 2026/06/22 05:45:47 INFO image resized width=1920 height=1080

Create a Custom Logger Instance

import { Logger } from '@hyperflake/slog';

const log = new Logger({ level: 'debug' });

log.debug('cache miss', { key: 'user:42' });

Log an Error with its Stack Trace

try {
    uploadImage();
} catch (err) {
    logger.error('upload failed', { folder: 123, image: 'abc.jpg', error: err });
}

2026/06/22 05:45:47 ERROR upload failed folder=123 image=abc.jpg error="connection refused\nat uploadImage (upload.ts:42:11)\nat processImage (worker.ts:88:5)"

The message and full stack trace are escaped onto that single line, so each error is still exactly one line in your log output (handy for log shippers, e.g. Docker's log driver, that read one line at a time).

Use JSON Output Instead of logfmt

import { Logger } from '@hyperflake/slog';

const log = new Logger({ format: 'json' });

log.info('image resized', { width: 1920, height: 1080 });
// {"time":"2026/06/22 05:45:47","level":"INFO","msg":"image resized","width":1920,"height":1080}

log.error('upload failed', { error: err });
// {"time":"2026/06/22 05:45:47","level":"ERROR","msg":"upload failed","error":{"message":"connection refused","stack":"Error: connection refused\n    at uploadImage (upload.ts:42:11)..."}}

Set a Global Default Format or Level

import { Logger, logger } from '@hyperflake/slog';

// Mirrors Go's slog.SetDefault: affects any Logger (including the
// exported `logger` singleton) that didn't explicitly set its own option.
Logger.setDefaults({ format: 'json', level: 'debug' });

logger.info('now emitted as JSON');

An explicit format/level passed to new Logger({...}) always takes priority over Logger.setDefaults().

Create a Child Logger with Persistent Fields

const requestLogger = logger.with({ requestId: 'r-123' });

requestLogger.info('handling request');
requestLogger.info('request completed', { status: 200 });

API

• new Logger(options?: { level?: LogLevel; format?: LogFormat; fields?: LogFields }): Creates a logger. level defaults to 'info', format defaults to 'logfmt'.
• logger.debug/info/warn/error(message: string, fields?: LogFields): Logs a line at the given level, gated by the logger's minimum level.
• logger.with(fields: LogFields): Returns a new Logger with the given fields merged into every future call. Does not mutate the original logger.
• Logger.setDefaults({ level?: LogLevel; format?: LogFormat }): Sets process-wide defaults used by any logger that didn't explicitly set that option itself (including the logger singleton), similar to Go's slog.SetDefault.
• logger (named export): A ready-to-use default Logger instance at level 'info'.

Examples

Here's a complete example combining child loggers and error logging:

import { logger } from '@hyperflake/slog';

const jobLogger = logger.with({ job: 'image-resize' });

jobLogger.info('job started');

try {
    resizeImage();
} catch (err) {
    jobLogger.error('job failed', { error: err });
}

Repository

Find the source code and contribute on GitHub.

License

This project is licensed under the ISC License - see the LICENSE file for details.