Logger

Class reference and examples for Logger — structured, level-based logging with transports and child loggers.

Import

import { Logger, createLogger } from "bytekit/logger";

Constructor

const logger = new Logger(config?);

Config options

Property	Type	Default	Description
level	"debug" | "info" | "warn" | "error"	"info"	Minimum severity level to emit
namespace	string	—	Prefix prepended to every message
formatter	(level, message, ...args) => string	—	Custom message formatting function
transports	Transport[]	—	Array of output targets
silent	boolean	false	When true, suppresses all output

Log levels

Levels are ordered by severity. Setting a level silences everything below it.

Level	Severity
debug	Lowest — verbose development output
info	General operational messages
warn	Potential issues worth investigating
error	Highest — failures that need attention

Methods

Method	Returns	Description
debug(message, ...args)	void	Logs at debug level
info(message, ...args)	void	Logs at info level
warn(message, ...args)	void	Logs at warn level
error(message, ...args)	void	Logs at error level
log(level, message, ...args)	void	Logs at an explicit level
setLevel(level)	void	Changes the minimum level at runtime
child(namespace)	Logger	Creates a child logger with a prefixed namespace
addTransport(transport)	void	Adds an output target at runtime
removeTransport(transport)	void	Removes a previously added transport

Transport interface

Any object that implements the following method can be used as a transport:

interface Transport {
  log(level: string, message: string, ...args: unknown[]): void;
}

createLogger factory

Shortcut that creates and returns a Logger instance. Accepts the same config as the constructor.

const logger = createLogger({ level: "debug", namespace: "app" });

Examples

Basic usage

import { Logger } from "bytekit/logger";

const logger = new Logger({ level: "info" });

logger.info("Server started on port %d", 3000);
logger.warn("Disk usage above 90%");
logger.error("Connection to database lost");

Child loggers

Create scoped loggers that inherit the parent’s config and prepend a namespace.

const root = new Logger({ level: "debug" });
const dbLogger = root.child("db");
const httpLogger = root.child("http");

dbLogger.info("Connected to postgres");  // [db] Connected to postgres
httpLogger.debug("GET /users 200 12ms"); // [http] GET /users 200 12ms

Custom transport

const fileTransport: Transport = {
  log(level, message, ...args) {
    const line = `[${new Date().toISOString()}] ${level}: ${message}\n`;
    fs.appendFileSync("app.log", line);
  },
};

const logger = new Logger({
  level: "info",
  transports: [fileTransport],
});

Runtime level switching

const logger = new Logger({ level: "info" });

// Later, enable debug output in response to a signal
process.on("SIGUSR2", () => {
  logger.setLevel("debug");
  logger.debug("Debug logging enabled");
});

Silent mode for tests

const logger = new Logger({ silent: true });

// No output — useful in test suites
logger.error("This is suppressed");

Use child() to give each module its own namespace. This makes it easy to filter logs in production without changing log levels globally.

The default transport writes to console. Add custom transports to send logs to files, remote services, or monitoring tools.