logger

Simple console logger that outputs json in prod and pretty messages on dev

Usage

npm install --save @5app/logger

const logger = require('@5app/logger');

logger.info('An email was sent', {
  email: '[email protected]',
  template: 'template1',
});

logger.error(new Error('Unknown playlist 123'));

Options

The logger can optionally be customised using the following environment variables:

• LOGS_FORMAT: if set to json, the logger will log messages in json format instead of pretty messages (default behaviour).
• LOGS_LEVEL: minimum logging level, by default it will be debug. Accepted values are 'debug', 'info', 'warn', and 'error'
• TAG: release tag (e.g. docker image tag) to be added to the log messages

Logging levels

Logging levels are (from lower to higher priority): 'debug', 'info', 'warn', and 'error'. The logger provides the logging functions with the following signatures: logger.<level>(message, objectOrError)

Here is an example of how the logger can be used:

logger.error('An error happened', new ApiError('The api call failed', 404)); // will log the message, the error message, the stack trace, and the statusCode error property
logger.warn('Be warned', {a: 1, b: Date.now(), c: 'some string'});
logger.info('An event happened', {a: 1, b: Date.now(), c: 'some string'});
logger.debug('A minor operation', {a: 1, b: Date.now(), c: 'some string'});

Slack alerts

logger.slackAlert([level], message, ...) logs like the regular level functions above, but prepends slackAlert to the message and adds a random slackAlertId (via crypto.randomUUID()) to the logged context, generating one even if no context is provided. level is optional and defaults to 'error'; when provided it must be one of 'debug', 'info', 'warn', or 'error'. Any remaining arguments are forwarded as-is to logger.<level>, so they follow that level's own signature (e.g. context and errorObject for 'error', just context for the others).

This is intended for hooking up a log-shipping pipeline (e.g. a log processor that watches for the slackAlert prefix) to post the message to Slack, using slackAlertId to correlate the Slack post back to the original log line, while still logging it at the given severity.

logger.slackAlert('Payment provider is down', error); // level defaults to "error"
logger.slackAlert('warn', 'Queue depth is high', {depth: 1200}); // context is merged with {slackAlertId: '...'}
logger.slackAlert('warn', 'Queue depth is high'); // context defaults to {slackAlertId: '...'}
logger.slackAlert('error', 'Payment provider is down', {provider: 'stripe'}, error);
logger.slackAlert('error', 'Payment provider is down', error); // the (message, errorObject) shorthand also works; slackAlertId is added alongside the error details

Note: because level is detected by checking if the first argument is a known level string, passing an unrecognised level (e.g. a typo like 'eror') is not an error — it's silently read as message instead, with level defaulting to 'error'.

Fetching context dynamically

In addition to providing a context object, you can also use logger.addContext to provide a function which will be called on every log to get a context object.

This can be helpful if you are using async_hooks or cls-hooked to store request metadata similarly to thread-local storage.

For instance, you can do the following without having to explicitly pass the correlationId to each log:

const asyncLocalStorage = new AsyncLocalStorage();

expressApp.use((req, res, next) {
  const correlationId = req.get('X-Correlation-Id') || uuidV4();
  asyncLocalStorage.run({correlationId}, () => next());
});

logger.addContext(() => asyncLocalStorage.getStore());
logger.info('User profile updated', {userId: 1234});

And the output will look like:

{"level":"info","timestamp":"2021-02-23T17:55:43.011Z","correlationId":"12bf5b37-e0b8-56e0-8dcf-dc8c4aefc123","userId":1234}