npm package: @open-draft/logger
Description: Environment-agnostic, ESM-friendly logger for simple needs.
Author: Artem Zakharchenko
Latest version: 0.3.0

# Logger

Environment-agnostic, ESM-friendly logger for simple needs.

## Why does this exist?

I've been using `debug` for quite some time but wanted to migrate my projects to better ESM support. Alas, `debug` doesn't ship as ESM so I went and wrote this little logger just for my needs. You will likely see it printing useful data in Mock Service Worker and beyond.

## Installation

```sh
npm install @open-draft/logger
```

## Usage

This package has the same API for both browser and Node.js and can run in those environments out of the box.

```js
// app.js
import { Logger } from '@open-draft/logger'

const logger = new Logger('parser')

logger.info('starting parsing...')
logger.warning('found legacy document format')
logger.success('parsed 120 documents!')
```

Logging is disabled by default. To enable logging, provide the `DEBUG` environment variable:

```sh
DEBUG=1 node ./app.js
```

> You can also use `true` instead of `1`. You can also use a specific logger's name to enable [logger filtering](#logger-filtering).

## API

- Class: `Logger`
  - [`new Logger(name)`](#new-loggername)
  - [`logger.debug(message, ...positionals)`](#loggerdebugmessage-positionals)
  - [`logger.info(message, ...positionals)`](#loggerinfomessage-positionals)
  - [`logger.success(message, ...positionals)`](#loggersuccessmessage-positionals)
  - [`logger.warning(message, ...positionals)`](#loggerwarningmessage-positionals)
  - [`logger.error(message, ...positionals)`](#loggererrormessage-positionals)
  - [`logger.extend(name)`](#loggerextendprefix)
  - [`logger.only(callback)`](#loggeronlycallback)

### `new Logger(name)`

- `name` `string` the name of the logger.

Creates a new instance of the logger. Each message printed by the logger will be prefixed with the given `name`. You can have multiple loggers with different names for different areas of your system.

```js
const logger = new Logger('parser')
```

> You can nest loggers via [`logger.extend()`](#loggerextendprefix).

### `logger.debug(message, ...positionals)`

- `message` `string`
- `positionals` `unknown[]`

Prints a debug message.

```js
logger.debug('no duplicates found, skipping...')
```

```
12:34:56:789 [parser] no duplicates found, skipping...
```

### `logger.info(message, ...positionals)`

- `message` `string`
- `positionals` `unknown[]`

Prints an info message.

```js
logger.info('new parse request')
```

```
12:34:56:789 [parser] new parse request
```

### `logger.success(message, ...positionals)`

- `message` `string`
- `positionals` `unknown[]`

Prints a success message.

```js
logger.success('prased 123 documents!')
```

```
12:34:56:789 ✔ [parser] prased 123 documents!
```

### `logger.warning(message, ...positionals)`

- `message` `string`
- `positionals` `unknown[]`

Prints a warning. In Node.js, prints it to `process.stderr`.

```js
logger.warning('found legacy document format')
```

```
12:34:56:789 ⚠ [parser] found legacy document format
```

### `logger.error(message, ...positionals)`

- `message` `string`
- `positionals` `unknown[]`

Prints an error. In Node.js, prints it to `process.stderr`.

```js
logger.error('failed to parse document')
```

```
12:34:56:789 ✖ [parser] failed to parse document
```

### `logger.extend(prefix)`

- `prefix` `string` Additional prefix to append to the logger's name.

Creates a new logger out of the current one.

```js
const logger = new Logger('parser')

function parseRequest(request) {
  const requestLogger = logger.extend(`${request.method} ${request.url}`)
  requestLogger.info('start parsing...')
}
```

```
12:34:56:789 [parser] [GET https://example.com] start parsing...
```

### `logger.only(callback)`

Executes a given callback only when the logging is activated. Useful for computing additional information for logs.

```js
logger.only(() => {
  const documentSize = getSizeBytes(document)
  logger.debug(`document size: ${documentSize}`)
})
```

> You can nest `logger.*` methods in the callback to `logger.only()`.

## Log levels

You can specify the log levels to print using the `LOG_LEVEL` environment variable.

There are the following log levels:

- `debug`
- `info`
- `success`
- `warning`
- `error`

> Providing no log level will print all the messages.

Here's an example of how to print only warnings:

```js
// app.js
import { Logger } from '@open-draft/logger'

const logger = new Logger('parser')

logger.info('some info')
logger.warning('some warning')
logger.error('some error')
```

```js
LOG_LEVEL=warning node ./app.js
```

```
12:34:56:789 ⚠ [parser] some warning
```

## Logger filtering

You can only print a specific logger by providing its name as the `DEBUG` environment variable.

```js
// app.js
import { Logger } from '@open-draft/logger'

const appLogger = new Logger('app')
const parserLogger = new Logger('parser')

appLogger.info('starting app...')
parserLogger.info('creating a new parser...')
```

```sh
DEBUG=app node ./app.js
```

```
12:34:56:789 [app] starting app...
```