write-logs.md

Write Logs

Call a level method to write a record. The logger supports the Winston npm levels: error, warn, info, http, verbose, debug, and silly.

import { createLogger } from "@eliware/log";

const logger = createLogger({ level: "debug" });
logger.info("Request received", { requestId: "demo", method: "GET" });
logger.debug("Cache lookup", { hit: true });

Add metadata

Pass an object as the second argument to add metadata fields. The logger places those fields beside the message in JSON output. The text format adds a JSON metadata object after the message.

logger.info("Request received", { requestId: "demo" });

Pass a primitive or array as the second argument to add it as value.

logger.info("Retry count", 3);
logger.info("Tags", ["blue", "green"]);

The logger treats these calls as { value: 3 } and { value: ["blue", "green"] }.

Add child context

Call child() to create a logger with shared context. The child logger adds that context to each record.

const requestLogger = logger.child({ requestId: "demo" });
requestLogger.info("Request received", { method: "GET" });

The record includes both requestId and method. The child logger uses the parent level, format, timestamp, transports, and redaction keys.

Read text output

The text format writes the level in uppercase brackets. It writes a string message as text. It adds metadata as JSON when metadata exists.

[INFO] Request received {"requestId":"demo"}

The formatter safely serializes non-string messages and metadata. See Safe serialization for value rules.

Read JSON output

The JSON format writes one JSON object for each record. It uses level and message fields. It adds metadata fields at the top level. If metadata uses level or message, the formatter keeps the log fields and places all metadata under metadata. It adds timestamp when you set timestamp: true.

{ "level": "info", "message": "Request received", "requestId": "demo" }

The formatter keeps string messages as text. It safely serializes non-string messages and metadata.

Use a custom transport

Pass a Winston transport in the transports array. Install Winston as a direct application dependency if your application creates its own transport. Check custom transport output for more fields or later format changes.