logger-core

A lightweight, dependency-free JSON logger for Node.js and TypeScript.

• 📄 Structured JSON logging
• 🎯 Configurable log levels
• ⚙️ Runtime configuration
• 🏷 Custom log field names
• 🔖 Constant fields
• 🚀 Fast and simple
• 💡 Written in TypeScript

Unlike traditional text loggers, every log entry is emitted as a JSON object, making it ideal for log aggregation systems such as Elasticsearch, Kibana, Splunk, Datadog, Google Cloud Logging, and AWS CloudWatch.

Installation

npm install logger-core

or

yarn add logger-core

Use Cases

• REST APIs
• Microservices
• Serverless Functions
• Docker Containers
• Kubernetes
• Cloud Run
• AWS Lambda
• Google Cloud Functions
• Azure Functions

Examples:

REST API

• sql-modular-sample: REST API with MySQL.
• mongo-simple-modular-sample: REST API with MongoDB.

Import Data

• import-sample: import a fix-length file to MySql.
• import-csv-sample: import a CSV file to MySql.

Export Data

• postgres-export-sample: export data from Postgres to CSV.
• mssql-export-sample: export data from MS SQL to CSV.
• mysql-export-sample: export data from MySql to CSV.

Message Queue

• rabbitmq-sample: An example to consume message from rabbitmq.
• activemq-sample: An example to consume message from activemq.
• nats-sample: An example to consume message from nats.

Quick Start

import { createLogger } from "logger-core";

const logger = createLogger({});

logger.info("Application started");
logger.warn("Low memory");
logger.error("Database unavailable");

Output

{
    "level":"info",
    "time":"2026-07-02T09:00:00.123Z",
    "msg":"Application started"
}

Configuration

import { createLogger } from "logger-core";

const logger = createLogger({
    level: "INFO"
});

Supported levels

| Level | Description | |--------|-------------| | TRACE | Everything | | DEBUG | Debug information | | INFO | General information | | WARN | Warning messages | | ERROR | Errors | | PANIC | Serious errors | | FATAL | Fatal errors |

Logging

logger.trace("trace");
logger.debug("debug");
logger.info("info");
logger.warn("warn");
logger.error("error");
logger.panic("panic");
logger.fatal("fatal");

Structured Data

Additional properties can be attached to every log.

logger.info("User login", {
    userId: 100,
    username: "john",
    ip: "192.168.1.10"
});

Output

{
    "level":"info",
    "time":"2026-07-02T10:00:00.123Z",
    "msg":"User login",
    "userId":100,
    "username":"john",
    "ip":"192.168.1.10"
}

Constant Fields

Fields that should appear in every log.

const logger = createLogger({
    constants: {
        service: "payment-service",
        version: "1.0.0",
        environment: "production"
    }
});

Output

{
    "service":"payment-service",
    "version":"1.0.0",
    "environment":"production",
    "level":"info",
    "time":"...",
    "msg":"Application started"
}

Custom Field Names

Default fields are

{
    "time":"...",
    "level":"INFO",
    "msg":"..."
}

They can be customized.

const logger = createLogger({
    map: {
        time: "@timestamp",
        level: "severity",
        msg: "message"
    }
});

Output

{
    "@timestamp":"...",
    "severity":"INFO",
    "message":"Application started"
}

Useful for Elasticsearch and Google Cloud Logging.

Custom Log Level Names

The displayed level names can be changed.

import { createLogger } from "logger-core";

const logger = createLogger({
    name: {
        trace: "TRACE",
        debug: "DEBUG",
        info: "INFORMATION",
        warn: "WARNING",
        error: "ERROR",
        panic: "PANIC",
        fatal: "FATAL"
    }
});

Output

{
    "level":"INFORMATION",
    "time":"...",
    "msg":"Application started"
}

Runtime Configuration

The logger configuration can be changed without restarting the application.

import * as http from "http";
import { createLogger, LogController } from "logger-core";

const logger = createLogger({
    level: "INFO"
});

const controller = new LogController(logger);

http.createServer(controller.config)
    .listen(8080);

Update log level

POST /config
Content-Type: application/json

{
    "level":"DEBUG"
}

Update field names

{
    "map":{
        "time":"@timestamp",
        "level":"severity",
        "msg":"message"
    }
}

Update constants

{
    "constants":{
        "service":"user-service",
        "environment":"production"
    }
}

Custom Output

Logs can be redirected anywhere.

const logger = createLogger(
    {},
    undefined,
    undefined,
    console.error,
    message => {

        // write to Kafka

        // write to Elasticsearch

        // write to file

        // send to Cloud Logging

        console.log(message);

    }
);

API

createLogger()

createLogger(config)

Creates a new logger.

Logger

logger.trace()
logger.debug()
logger.info()
logger.warn()
logger.error()
logger.panic()
logger.fatal()

isXXXEnabled()

logger.isTraceEnabled()

logger.isDebugEnabled()

logger.isInfoEnabled()

logger.isWarnEnabled()

logger.isErrorEnabled()

logger.isPanicEnabled()

logger.isFatalEnabled()

Useful for expensive logging.

if (logger.isDebugEnabled()) {

    logger.debug(
        expensiveCalculation()
    );

}

Example Output

{
    "service":"payment-service",
    "version":"1.2.0",
    "environment":"production",
    "level":"INFO",
    "time":"2026-07-02T09:18:10.123Z",
    "msg":"Order created",
    "orderId":100,
    "userId":55
}

Features

• Lightweight
• Zero runtime dependencies
• TypeScript support
• Structured JSON logs
• Runtime configuration
• Custom log levels
• Custom field names
• Constant fields
• Pluggable output
• Node.js compatible

License

MIT