Quick start

Setting up

Set up LogTape in the entry point of your application using configure():

import { configure, getConsoleSink } from "@logtape/logtape";

await configure({
  sinks: { console: getConsoleSink() },
  loggers: [
    { category: "my-app", lowestLevel: "debug", sinks: ["console"] }
  ]
});

WARNING

If you are composing a library, you should not set up LogTape in the library itself. It is up to the application to set up LogTape.

See also Using in libraries.

And then you can use LogTape in your application or library:

import { getLogger } from "@logtape/logtape";

const logger = getLogger(["my-app", "my-module"]);

export function myFunc(value: number): void {
  logger.debug `Hello, ${value}!`;
}

For detailed information, see Configuration.

How to log

There are total 5 log levels: debug, info, warning, error, fatal (in the order of verbosity). You can log messages with the following syntax:

logger.debug `This is a debug message with ${value}.`;
logger.info  `This is an info message with ${value}.`;
logger.warn  `This is a warning message with ${value}.`;
logger.error `This is an error message with ${value}.`;
logger.fatal `This is a fatal message with ${value}.`;

Structured logging

You can also log messages with a function call. In this case, log messages are structured data:

logger.debug("This is a debug message with {value}.", { value });
logger.info("This is an info message with {value}.", { value });
logger.warn("This is a warning message with {value}.", { value });
logger.error("This is an error message with {value}.", { value });
logger.fatal("This is a fatal message with {value}.", { value });

For detailed information, see Structured logging.

Lazy evaluation

Sometimes, values to be logged are expensive to compute. In such cases, you can use a function to defer the computation so that it is only computed when the log message is actually logged:

logger.debug(l => l`This is a debug message with ${computeValue()}.`);
logger.debug("Or you can use a function call: {value}.", () => {
  return { value: computeValue() };
});