@yingyeothon/logger

Minimal structured logger with severity filtering, writer composition, and pluggable log writers. Built for tiny serverless bundles: zero dependencies, a few hundred bytes of code.

One threshold, then any number of writers; an omitted logger option becomes nullLogger.

flowchart LR
  A["logger.info(message, context)"] --> F["createFilteredLogger<br/>severity threshold"]
  F --> C["combine"]
  C --> W1["consoleWriter"]
  C --> W2["your own LogWriter"]
  N["nullLogger"] -.->|"the default when logger is omitted"| A

Install

npm install @yingyeothon/logger

Usage

ESM:

import {
  combine,
  createConsoleLogger,
  createFilteredLogger,
  nullLogger,
} from "@yingyeothon/logger";

const logger = createConsoleLogger("info");
logger.debug("dropped: below severity");
logger.info("hello", { requestId: "r-1" });
logger.error("boom", new Error("bad"));

// Compose multiple writers and filter them together.
const buffered: string[] = [];
const audit = createFilteredLogger({
  severity: "debug",
  writer: combine(
    {
      debug: () => {},
      info: (...a) => buffered.push(a.join(" ")),
      warn: console.warn,
      error: console.error,
    },
    nullLogger, // combine() skips the null logger
  ),
});
audit.info("stored");

CJS:

const { createConsoleLogger } = require("@yingyeothon/logger");
const logger = createConsoleLogger("debug");
logger.debug("visible now");

Public API

• createConsoleLogger(severity?) — Logger filtering the console writer (default severity "info")
• createFilteredLogger(options) — Logger that writes to options.writer only when the record's severity is at or above the logger's (mutable) severity
• combine(...writers) — fan-out writer; skips nullLogger
• consoleWriter — LogWriter backed by console.debug/info/warn/error
• nullLogger — a Logger that drops everything
• Types: LogSeverity ("none" | "debug" | "info" | "warn" | "error"), LogWriter ({ debug, info, warn, error } sink interface), Logger (LogWriter plus a mutable severity field), FilteredLoggerOptions ({ severity, writer })

Migrating from the legacy package

• The exported classes are gone: new ConsoleLogger(severity) becomes createConsoleLogger(severity) and new FilteredLogger(severity, writer) becomes createFilteredLogger({ severity, writer }). The returned objects are plain Logger values with the same behavior, including the mutable severity field.
• warn was added to the contract: LogWriter sinks must provide debug/info/warn/error, and LogSeverity gained "warn".
• The package now ships dual ESM/CJS with types; deep imports (@yingyeothon/logger/lib/...) are no longer supported — import everything from the package root.
• Logger, LogSeverity, and LogWriter are type-only exports.